Section 23. The frog has an ovum with a moderate allowance of yolk, but the quantity is only sufficient to start the little animal a part of its way towards the adult state. The fowl, on the contrary, has an enormous ovum, gorged excessively, with yolk, and as a consequence the chick is almost perfected when it is hatched. The so-called yolk, the yellow of an egg, is the ovum proper; around that is a coating of white albumen, in a shell membrane and a shell. At either end of the yolk (Figure 1, y.) twisted strands of albuminous matter, thechalazae(ch.) keep the yolk in place. The animal pole is a small grey protoplasmic area, thegerminal area(g.a.), on the yolk.
Section 24. We pointed out that the presence of the yolk in the frog's egg led to a difference in the size of the cells at the animal and vegetable poles. The late F.M. Balfour, borrowing a mathematical technicality, suggested that the rate of segmentation in any part of an ovum varies inversely with the amount of yolk. In the fowl's egg, except just at the germinal area, the active protoplasm is at a minimum, the inert yolk at a maximum; the ratio of yolk to protoplasm is practically infinity, and the yolk therefore does not segment at all. The yolk has diluted the active protoplasm so much as to render its influence inappreciable. The germinal area segments, and lies upon the yolk which has defeated the efforts of its small mingling of protoplasm to divide. Such a type of segmentation in which only part of the ovum segments is calledmeroblastic. If we compare this with the typicalblastosphereof the lower type, we see that it is, as it were, flattened out on the yolk. This stage is shown in section in the lower figure ofFigure 1. b.d., theblastoderm, is from this point of view, a part of the ripped and flattened blastosphere, spread out on the yolk; s.c. is the segmentation cavity, and y. the yolk.
Section 25. There is no open invagination of an archenteron in the fowl, as in the frog--, the gastrula, like the blastosphere, stage is also masked. But, in the hinder region of the germinal area, a thick mass of cells, grows inward and forward, and, appearing in the dorsal view of the egg as a white streak, is called theprimitive streak(p.s.). By a comparison of the figures of frog and fowl the student will easily perceive the complete correspondence of the position of this with the blastopore of the frog. The relation of the two will be easily understood if we compare the fowl's archenteron to a glove-finger under pressure-- its cavity is obliterated-- and the frog's to the glove-finger blown out. The tension of the protoplasm, straining over the enormous yolk, answers to the pressure. The gastrula in the fowl is solid. The primitive streak is, in fact,the scar of a closed blastopore. As we should expect from this view of its homology, at the primitive streak, the three embryonic layers are continuous and indistinguishable (Figure 2). Elsewhere in the blastoderm they are distinctly separate. Just as the yolk cells of the frog form the ventral wall of the intestine, so nuclei appear along the upper side of the yolk of the fowl, where some protoplasm still exists, and give rise to the ventral hypoblastic cells. By conceiving a gradually increasing amount of yolk in the hypoblastic cells in the ventral side of the archenteron, the substantial identity of the gastrula stage in the three types, which at first appear so strikingly different, will be perceived. CarryFigures 4 and 5of the frog one step further by increasing the size of the shaded yolk and leaving it unsegmented, and instead of ar. in 5 show a solid mass of cells, and the condition of things in the fowl would at once be rendered.
Section 26.Figure 3aof the fowl will conveniently serve for comparison withFigure 7of the frog. The inturning of the medullary groove is entirely similar in the two cases. The mesoblast appears as solid mesoblastic somites. In the section aboveFigure 4this layer is shown as having split into somatopleur (so.) and splanchnopleur (spch.).Figure 3answers toFigure 6of the frog, andFigure 4is a later stage, in which the medullary groove is beginning to close at its middle part. The clear club-shaped area around the embryo (a.p.) is thearea pellucida; the larger area without this is thearea opaca(a.o.), in which the first bloodvessels arise by a running together and a specialization of cells. The entire germinal area grows steadily at its edges to creep over and enclose the yolk.
Section 27. So far, the essential differences between the development of fowl and frog, themeroblasticsegmentation, absence of a typical gastrula, and the primitive streak, seem comprehensible on the theory that such differences are due to the presence of an enormous amount of yolk. Another difference that appears later is that, while the tadpole has an efficient pronephros, the fowl, which has no larval (free imperfect) stages in its life history, has the merest indication of such a structure.
Section 28. Another striking contrast, due to, or connected with, this plethora of yolk, is the differentiation of a yolk sac (= umbilical vesicle) and the development of two new structures, theamnionandallantois, in the fowl. If the student will compareFigure 10of the frog, he will see that the developing tadpole encloses in its abdomen all the yolk provided for it. This is a physical impossibility in the fowl. In the fowl (Figure 2,Sheet 24) the enormous yolk (Y.) lies outside of the embryo, and, as the cells of the germinal area grow slowly over it,umbilical bloodvesselsare developed to absorb and carry the material to the embryo. In the case of an embryo sinking in upon, as it absorbs, this mass of nutritive material, a necessity for some respiratory structure is evident. From the hinder end of the fowl's intestine, in a position corresponding to the so-called, urinary bladder of the frog, a solid outgrowth, the allantois, which speedily becomes hollow, appears. Early stages are shown in Figures 1 and 2,Sheet 24(al.); while the same thing is shown more diagrammatically onSheet 23, Figure 6 (all.). This becomes at last a great hollow sac, which is applied closely to the porous shell, and the extent of which will be appreciated by looking at Figure 5,Sheet 24, where the allantois is shaded. Allantoic bloodvessels ramify thickly over its walls, and aeration occurs through the permeable shell.
Section 29. The nature of theamnionwill be understood by following Figures 4b, 5, and 6 onSheet 23. The three embryonic layers are indicated by broken lines, dots, and black lines, just as they are in the frog diagrams. Not only is the embryo slowly pinched off from the yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows in size, it sinks into the space thus made, the extra-embryonic somatopleur and epiblast rise up round it as two folds, which are seen closing in5, and closed in6, over the dorsal side of the young chick. In this way a cavity, a., lined by epiblast, and called theamniotic cavity, is formed. Dorsal to this, in6,comes a space lined by somatic mesoblast, and continuous with p.p., the pleuro-peritoneal cavity, or body cavity of the embryo. Outside this, again, is a layer, of somatopleur internally and epiblast externally, thefalse amnion(f.a.), which is continuous with theserous membrane(s.m.) enclosing the rest of the egg. The student should, carefully copy these diagrams, with coloured pencils or inks for the different layers, and should compare them with the more realistic renderings of Figures 2, 5, and 8,Sheet 24.
Section 30. The heart in the fowl appears first as apair of vessels, which unite to form a straight trunk in the median line, as the flattened-out embryo closes in from the yolk. The way in which this straight trunk is thrown, first of all, into theSshape of the fish heart, and then gradually assumes the adult form, is indicated roughly byFigure 3. In one respect the development of the heart does not follow the lines one would expect. Since, between the fish and the higher form comes the condition of such an animal as the frog, in which the auricles are divided, while there is only one ventricle, we might expect a stage in which the developing chick's heart would have one ventricle, and a septum between the auricles. But, as a matter of fact, the ventricles in fowl and rabbit are separated first, and the separation of the auricles follows, and is barely complete at birth.
Section 31. Twovitelline veinsfrom the yolk sac (v.v.) flow into the heart from behind, as shown inFigure 1. A later more complete and more diagrammatic figure of the circulation is seen inFigure 7. At first there are twoanterior cardinal(a.c.), and twoposterior cardinal veins(p.c.) uniting to form Cuvierian sinuses (c.s.) that open into the heart just as in the dog-fish. But later the inferior cava is developed and extends backward, the posterior cardinals atrophy, the Cuvierian sinuses become the superior cavae, and the anterior cardinals the internal jugular veins. The vitelline veins (v.v.) flow, at first, uninterruptedly through the liver to the inferior cava, but, as development proceeds, a capillary system is established in the liver, and the through communication, theductus venosus, is reduced-- at last-- completely. Bearing in mind that the yolk is outside the body in the fowl and inside it in the frog, the vitelline veins of the former have a considerable resemblance in position, and in their relation to the portal vein, to a portion of the single anterior abdominal vein. Blood is taken out to the allantois, however, by the arteries of the latter type.
Section 32. Five aortic arches are generally stated to appear altogether in the fowl, but not simultaneously. The first two, the mandibular and the hyoid vascular arches, early disappear, and are not comparable to any in the frog. The third is the first branchial arch, and, like the corresponding arch in the frog, forms the carotid artery; the second branchial is the aortic arch; and what has hitherto been regarded as the third (thefiftharch, i.e.) the pulmonary artery. A transitory arch, it is now known, however, appears between the second branchial and the last, and it is therefore the fourth branchial arch which is the pulmonary, just as it is in the frog.
Section 33. Blood, it may be mentioned, first appears in thearea vasculosa, the outer portion of thearea opaca. Embryonic cells send out processes, and so become multipolar; the processes of adjacent cells coalesce. The nucleus divides, and empty spaces appear in the substance of each of the cells.
In this way, the cavities of the smaller vessels and capillaries are formed, and the products of the internal divisions of the cells become the corpuscles within the vessels. The red blood corpuscles of the rabbit, it may be added, arenucleatedfor a considerable portion of embryonic life. Larger vessels and the heart are burrowed, as it were, out of masses of mesoblast cells. The course of the blood in the embryo is by the veins to the right auricle,thence through the imperfection of the auricular septum already alluded to, into the left auricle. Then the left ventricle, aortic arches (for the future pulmonary artery is in communication by a part presently blocked, theductus arterious, with the systemic aorta), arteries, capillaries, veins. The liver capillary system and the pulmonary system only become inserted upon the circulation at a comparatively late stage.
Section 34. With the exception of the reduction of the pronephros, what has been said of the development of the frog's nervous system, renal and reproductive organs, and skeleton, applies sufficiently to the fowl for our present purposes. The entire separation of Wolffian and Mullerian ducts from the very beginning of development is here beyond all question (videSection 18). But the notochord in the fowl is not so distinctly connected with the hypoblast, and so distinct from the mesoblast, as it is in the lower type, and no gills, internal or external, are ever developed. The gillslitsoccur with a modification due to the slitting and flattening out of the embryo, already insisted upon; for, whereas in the tadpole they may be described as perforations, in the fowl they appear as four notches between ingrowing processes that are endeavouring to meet in the middle line.
Section 35. The early development of the rabbit is apt to puzzle students a little at first. We have an ovum practically free from yolk (alecithal), and, therefore, we find it dividing completely and almost equally. We naturally assume, from what we have learnt, that the next stages will be the formation of a hollow blastosphere, invagination, a gastrula forming mesoblast by hollow outgrowths from the archenteron, and so on. There is no yolk here to substitute epiboly (Section 9) for invagination, nor to obliterate the archenteron and the blastopore through its pressure.
Yet none of these things we have anticipated occur!
We findsolidmesoblastic somites, we find primitive streak, allantois and amnion, features we have just been explaining as the consequence of an excess of yolk in the egg. We even find a yolk sac with no yolk in it.
Section 36. A solid mass of cells is formed at the beginning, called amorula, Figure 1. In this we are able to distinguish rather smallerouter layer cells(o.l.c.), and rather largerinner layer cells(i.l.c.), but these cells, in their later development, do not answer at all to the two primitive layers of the gastrula, and the name ofVan Beneden's blastopore(V.B.b.), for a point where the outer layer of cells is incomplete over the inner, only commemorates the authorship of a misnomer. The uniformity, or agreement, in the development of our other vertebrate types is apparently departed from here.
Illustration: Development Section 36.Illustration: Development Section 36.
Illustration: Development Section 36.Illustration: Development Section 36.
Section 37. As the egg develops, however, we are astonished to find an increasing resemblance to that of the fowl. A split occurs at one point between outer layer and inner layer cells, and the space resulting (Y in Figure 2) is filled by an increasing amount of fluid, and rapidly enlarges, so that presently we have the state of affairs shown in 3, in which the inner layer cells are gathered together at one point on the surface of the ovum, and constitute the germinal area. If, with Hubrecht, we regard the outer layer cells as an egg membrane, there is a curious parallelism between this egg and the fowl's the fluid Y representing the yolk; and the inner layer cells the cells of the fowl's germinal area.
At any rate, the subsequent development goes far to justify such a view. The inner cells split into epi-, meso-, and hypo-blast, like the blastoderm in the fowl; there is a primitive streak and no blastopore; an amnion arises; the yolk sac, small and full of serous fluid, is cut off just as the enormous yolk of the fowl is cut off; and an allantois arises in the same way. There is no need to give special diagrams--Figures 3, 4b, 5, and 6of the fowl will do in all respects, except proportion, for the development of the rabbit. The differences are such as we may account for, not on the supposition that the rabbit's ovum never had any yolk, but that an abundant yolk has been withdrawn from it. The nutrition of the embryo by yolk has been superseded by some better method. The supposition that the rabbit is descended from ancestors which, like the birds and reptiles, laid eggs with huge quantities of yolk, meets every circumstance of the case.
Section 38. But the allantois and yolk sac of the rabbit, though they correspond in development, differ entirely in function from the similar organs of the fowl. The yolk sac is of the very smallest nutritive value; instead of being the sole source of food, its contents scarcely avail the young rabbit at all as nourishment. Its presence in development is difficult to account for except on the supposition, that it was once of far greater importance. At an early stage, the outgrowing allantois, pushing in front of it the serous membrane, is closely applied to the lining of the mother's uterus. The maternal uterus and the embryonic allantois send out finger-like processes into each other which interlock, and the tissue between the abundant bloodvessels in them thins down to such an extent that nutritive material, peptones and carbohydrates, and oxygen also, diffuse freely through it from mother tofoetus,* and carbon dioxide, water, and urea from thefoetusto the mother. The structure thus formed by the union of the wall of the maternal uterus, allantois, and the intermediate structures is called theplacenta. Through its intermediation, the young rabbit becomes, as it were, rooted andparasiticon the mother, and utilizes her organs for its own alimentation, respiration, and excretion. It gives off CO2, H2O, and urea, by the placenta, and it receives O and elaborated food material through the same organ. This is the better method that has superseded the yolk.
* The embryo.
Section 39. In its later development, the general facts already enunciated with regard to the organs of frog and fowl hold, and where frog and fowl are stated to differ, the rabbit follows the fowl. In the circulation the left fourth vascular arch (second branchial) gives rise to the aortic arch; in the right the corresponding arch disappears, except so much of it as remains as the innominate artery. The azygos vein (Chapter 3) -is a vestige of- [is derived from] the right posterior cardinal sinus. Both pulmonary arteries in the rabbit are derived from theleftsixth vascular arch (= fourth branchial). CompareSection 32. The allantois altogether disappears in the adult fowl; in the adult mammal a portion of its hollow stalk remains as the urinary bladder, and the point where it left the body is marked by theumbilicusornavel. The umbilical arteries become the small hypogastric arteries on either side of the urinary bladder. There is no trace of a pronephros at all in the rabbit.
Section 40. We may note here the development of the eye. This is shown in Figure 4,Sheet 24. A hollow cup-shaped vesicle from the brain grows out towards an at first hollow cellular ingrowth from the epidermis. The cavity within the wall of the cup derived from the brain is obliterated, [and the stalk withers,] the cup becomes the retina, and -its stalk- [thence fibres grow back to the brain to form] the optic nerve. The cellular ingrowth is the lens. The remainder of the eye-structures are of mesoblastic origin, except the superficial epithelium of the cornea. The retinal cup is not complete at first along the ventral line, so that the rim of the cup, viewed as inFigure 1, r., is horseshoe shaped. -Hence the optic nerve differs from other nerves in being primitively hollow.- In all other sense organs, as, for instance, the olfactory sacs and the ears, the percipient epithelium is derived, from the epiblast directly, and not indirectly through the nervous system. These remarks apply to all vertebrate types.
Section 41. The supposition, that the general characters of the rabbit's ovum were stamped upon it as an heritage from a period when the ancestors of the mammals were egg-laying reptiles, is strengthened by the fact that the two lowest and most reptile-like of all the mammalia, the duck-billed platypus and the echidna, have been shown to depart from the distinctive mammalian character, and to lay eggs. And, in further confirmation of this supposition, we find, in tracing the mammals and reptiles back through the geological record, that in the Permian and Triassic rocks there occur central forms which combine, in a most remarkable way, reptilian and mammalian characteristics.
Section 42. In conclusion, we would earnestly recommend the student toseemore of embryological fact than what is given him here. It is seeing and thinking, much more than reading, which will enable him to clothe the bare terms and phrases of embryology with coherent knowledge. In Howes'Atlas of Biologythere is a much fuller series of figures of the frog's development than can be given here, and they are drawn by an abler hand than mine can pretend to be. There is also anAtlas d'Embryologie, by Mathias Duval, that makes the study of the fowl's development entertaining and altogether delightful. Such complete series as these are, from the nature of the case, impossible with the rabbit. Many students who take up the subject of biology do so only as an accessory to more extended work in other departments of science. To such, practical work in embryology is either altogether impossible, or only possibly to a very limited extent. The time it will consume is much greater, and the intellectual result is likely to be far less than the study of such plates as we have named.
Section 43. We have now considered our types, both from the standpoint of adult anatomy and from embryological data; and we have seen through the vertebrate series a common structure underlying wide diversity in external appearance and detailed anatomy. We have seen a certain intermediateness of structure in the frog, as compared with the rabbit and dog-fish, notably in the skull and skeleton, in the circulation, in the ear, and in the reduced myomeres; and we have seen that the rabbit passes in these respects, and in others, through dog-fish- and frog-like stages in its development, and this alone would be quite sufficient to suggest that the similarities of structure are due to other causes than a primordial adaptation to certain conditions of life.
Section 44. It has been suggested by very excellent people that these resemblances are due to some unexplained necessity of adherence totype, as though, the power that they assume created these animals originally, as they are now, coupled creative ability with a plentiful lack of ideas, and so perforce repeated itself with impotent variations. On the other hand, we have the supposition that these are "family likenesses," and the marks of acommon ancestry. This is the opinion now accepted by all zoologists of repute.
Section 45. It must not be for a moment imagined that it is implied that rabbits are descended from frogs, or frogs from dog-fish, but that these three forms are remote cousins, derived from some ancient and far simpler progenitor. But since both rabbit and frog pass through phases like the adult condition of the dog-fish, it seems probable that the dog-fish has remained more like the primordial form than these two, and similarly, the frog than the rabbit.
Section 46. Hence we may infer that the mammals were the last of the three groups, of which we have taken types, to appear upon the earth, and that the fishes preceded, the amphibia. Workers in an entirely independent province, that of palaeontology, completely endorse this supposition. The first Vertebrata to appear in the fossil history of the world are fishes; fish spines andplacoid scales(compare dog-fish) appear in the Ordovician rocks. In the coal measures come the amphibia; and in the Permo-triassic strata, reptile-like mammals. In the Devonian rocks, which come between the Silurian and the coal measures, we find very plentiful remains of certain fish called thedipnoi, of which group three genera still survive; they display, in numberless features of their anatomy, transitional characters between true fish and amphibia. Similarly, in the Permian come mammal-like reptiles, that point also downward to the amphibia. We find, therefore, the story told by the ovum written also in the rocks.
Section 47. Now, when this fact of a common ancestry is considered, it becomes necessary to explain how this gradual change of animal forms may have been brought about.
Section 48. Two subcontrary propositions hold of the young of any animal. It resembles in many points its parent. It differs in many points from its parent. The general scheme of structure and the greater lines of feature are parental, inherited; there are also novel and unique details that mark the individual. The first fact is the law ofinheritance; the second, ofvariation.
Section 49. Now the parent or parents, since they live and breed, must be more or less, but sufficiently, adapted to their conditions of living-- more or less fitted to the needs of life. Thevariationin the young animal will be one of three kinds: it will fit the animal still better to the conditions under which its kind live, or it will be a change for the worse, or it is possible to imagine that the variation-- as in the colour variations of domesticated cats-- will affect its prospects in life very little. In the first case, the probability is that the new animal will get on in life, and breed, and multiply above the average; in the second, it is probable that, in the competition for food and other amenities of life, the disadvantage, whatever it is, under which the animal suffers will shorten its career, and abbreviate the tale of its offspring; while, in the third case, an average career may be expected. Hence, disregarding accidents, which may be eliminated from the problem by taking many cases, there is a continual tendency among the members of a species of animals in favour of the proportionate increase of the individuals most completely adapted to the conditions under which the species lives. That is, while the conditions remain unchanged, the animals, considered as one group, are continually more highly perfected to live under those conditions. And under changed conditions the specific form will also change.
Section 50. The idea of this process of change may be perhaps rendered more vivid by giving an imaginary concrete instance of its working. In the jungles of India, which preserve a state of things which has existed for immemorial years, we find the tiger, his stripes simulating jungle reeds, his noiseless approach learnt from nature in countless millions of lessons of success and failure, his perfectly powerful claws and execution methods; and, living in the same jungle, and withhimas one of the conditions of life, are small deer, alert, swift, light of build, inconspicuous of colour, sharp of hearing, keen-eyed, keen-scented-- because any downward variation from these attributes means swift and certain death. To capture the deer is a condition, of the tiger's life, to escape the tiger a condition of the deer's; and they play a great contest under these conditions, with life as the stake. The most alert deer almost always escape; the least so, perish.
Section 51. But conditions may alter. For instance, while most of these deer still live in the jungle with tigers, over a considerable area of their habitat, some change may be at work that thins the jungle, destroys the tigers in it, and brings in, let us say, wolves, as an enemy to the deer, instead of tigers. Now, against the wolves, which do not creep, but hunt noisily, and which do not spring suddenly upon prey, but follow by scent, and run it down in packs, keen eyes, sharp ears, acute perceptions, will be far less important than endurance in running. The deer, under the new conditions, will need coarser and more powerful limbs, and a larger chest; it will be an advantage to be rough and big, instead, of frail and inconspicuous, and the ears and eyes need not be so large. The old refinements will mean weakness and death; any variation along the line of size and coarseness will be advantageous. Slight and delicate deer will be continually being killed, rougher and stronger deer continually escaping. And so gradually, under the new circumstances, if they are not sufficient to exterminate the species, the finer characteristics will be eliminated, and a new variety of our old jungle deer will arise, and, if the separation and contrast of the conditions is sufficiently great and permanent, we may, at last, in the course of ages, get a new kind of deer specifically different in its limbs, body, sense organs, colour, and instincts, from the deer that live in the jungle. And these latter will, on their side, be still continually more perfected to the jungle life they are leading.
Section 52. Take a wider range of time and vaster changes of condition than this, and it becomes possible to imagine how the social cattle-- with their united front against an enemy, fierce onslaught, and their general adaptation to prairie life-- have differentiated from the ancestors of the slight and timid deer; how the patient camel, with his storage hump, water storage, and feet padded against hot sand, has been moulded by the necessity of desert life from the same ancestral form. And so we may work back, and link these forms, and other purely vegetarian feeders, with remoter cousins, the ancestral hogs. Working in this way, we presently get a glimpse of a possible yet remoter connection of all these hoofed and mainly vegetarian animals, with certain "central types" that carry us across to the omnivorous, and, in some cases, almost entirely vegetarian bears, and to the great and prosperous family of clawed, meat-eaters. And thus we elucidate, at last, a thread of blood relationship between the, at present, strongly contrasted and antagonistic deer and tiger, and passing thence into still wider generalizations, it would be possible to connect the rabbit playing in the sunshine, with the frog in the ditch, the dog-fish in the sea-waters and the lancelet in the sand. For the transition from dog-fish to rabbit differs from the transition from one species of deer to another only in magnitude: it is an affair of vast epochs instead merely of thousands of years.
Section 53. It would, however, be beyond the design of this book to carry our demonstration of the credibility of a common ancestry of animals still further back. But we may point out here that it is not a theory, based merely upon one set of facts, but one singularly rich in confirmation. We can construct, on purely anatomical grounds, a theoretical pedigree. Now the independent study of embryology suggests exactly the same pedigree, and the entirely independent testimony of palaeontology is precisely in harmony with the already confirmed theory arrived at in this way.
Section 54. It is in the demonstration of this wonderful unity in life, only the more confirmed the more exhaustive our analysis becomes, that the educational value and human interest of biology chiefly lies. In the place of disconnected species of animals, arbitrarily created, and a belief in the settled inexplicable, the student finds an enlightening realization of uniform and active causes beneath an apparent diversity. And the world is not made and dead like a cardboard model or a child's toy, but a living equilibrium; and every day and every hour, every living thing is being weighed in the balance and found sufficient or wanting.
Our little book is the merest beginning in zoology; we have stated one or two groups of facts and made one or two suggestions. The great things of the science of Darwin, Huxley, Wallace, and Balfour remain mainly untold. In the book of nature there are written, for instance, the triumphs of survival, the tragedy of death and extinction, the tragi-comedy of degradation and inheritance, the gruesome lesson of parasitism, and the political satire of colonial organisms. Zoology is, indeed, a philosophy and a literature to those who can read its symbols. In the contemplation of beauty of form and of mechanical beauty, and in the intellectual delight of tracing and elucidating relationships and criticising appearances, there is also for many a great reward in zoological study. With an increasing knowledge of the facts of the form of life, there gradually appears to the student the realization of an entire unity shaped out by their countless, and often beautiful, diversity. And at last, in the place of the manifoldness of a fair or a marine store, the student of science perceives the infinite variety of one consistent and comprehensive Being-- a realization to which no other study leads him at present so surely.
To the student who feels inclined to amplify this brief outline of Vertebrate Anatomy, we may mention the following books: Wiedersheim's and Parker'sVertebrates, Huxley'sAnatomy of the Vertebrata, Flower'sOsteology of the Mammalia, Wallace'sDistribution, Nicholson and Lyddeker'sPalaeontology(Volume 2), the summaries in Rolleston'sForms of Animal Life(where a bibliography will be found), and Balfour'sEmbryology. But reading without practical work is a dull and unprofitable method of study.
[All these questions were actually set at London University Examinations.]{In Both Editions.}
-Miscellaneous Questions._
-Miscellaneous Questions._
[Most of these questions were actually set at the Biological Examinations of London University.]{In Both Editions.}
-Note on Making Comparisons._
-Note on Making Comparisons._
Students preparing for examinations are frequently troubled by "comparison" questions. Tabulation is often recommended, but we are inclined to favour a rather more flexible plan of marking off differences and resemblances. In tabulation a considerable loss of time is occasioned by writing down the features of both the things compared, and this is a serious consideration for the examinee. We advise him therefore, first, if he possibly can, to draw side by side and in corresponding positions the two things under consideration, and then, going over them in a methodical way, to state simply the difference between each homologous part. We append as examples three test answers actually submitted (with figures) in "Correspondence" work:--
1. Compare the brain of the frog with that of the rabbit.
In the frog'sfore-brain--The olfactory lobes are fused in the middle line.There is no corpus callosum, nor is there a middle commissure to the third ventricle.The cerebral hemispheres are not convoluted, and, looked at from the dorsal aspect, do not hide the thalamencephalon and mid-brain.The pineal gland lies in the cranial wall and not deeply between the hemispheres, and its stalk is longer and tilts forward.In themid-brain--The optic lobes are two, instead of being corpora quadrigemina, and hollow.In thehind-brain--The cerebellum is a very small transverse band, and has no lateral parts.The medulla is relatively larger.There are no spinal accessory nor hypoglossal nerves to the brain.
The olfactory lobes are fused in the middle line.There is no corpus callosum, nor is there a middle commissure to the third ventricle.The cerebral hemispheres are not convoluted, and, looked at from the dorsal aspect, do not hide the thalamencephalon and mid-brain.The pineal gland lies in the cranial wall and not deeply between the hemispheres, and its stalk is longer and tilts forward.
The optic lobes are two, instead of being corpora quadrigemina, and hollow.
The cerebellum is a very small transverse band, and has no lateral parts.The medulla is relatively larger.There are no spinal accessory nor hypoglossal nerves to the brain.
2. Compare the vertebrae of dog-fish, rabbit, and frog.
The centra of the dog-fish are -opistho- [amphi]-coelous (i.e., hollow at either end).The centra of the rabbit are flat-faced.The centra of the frog are procoelous (hollow in front).The notochord persistsbetweenthe centra in the dog-fish and rabbit,withinthe centra in frog.The centra of the rabbit have epiphyses, absent in the dogfish and frog.The transverse processes of the rabbit typically bear ribs. Short ribs occur in the dog-fish, but their homology with those of the rabbit is doubtful. The frog has no ribs.The interneural plates are peculiar to the dog-fish in this comparison.
3. Compare the skull of the dog with that of the frog.
The Brain Case--Of the frog is a cylindrical box, from which the otic capsules project conspicuously on either side. It contains only two ossifications in its cartilaginous substance (the sphen-ethmoid and the ex-occipital), being protected by the membrane bones, the parieto-frontals above and the parasphenoid below.In the mammal it is enormously inflated, and the otic capsules are imbedded in its wall. There are supra- and basi- as well as ex-occipital bones; the para-sphenoid is (? entirely) gone, and its place is taken by the basi- and pre-sphenoids, and the lateral walls contain fresh paired ossifications, the ali- and orbito-sphenoids-- all cartilage bones. The sphenethmoid is perhaps represented in part by the ethmoid.As a result of the inflation of the brain-case, the squamosal, which slopes downward and outward in the frog, and overlies the cartilaginous suspensorium (quadrate cartilage), has become a constituent of the brain-case wall, and slopes downwardly and in.Jaw Suspension--The point of attachment of the jaw has shifted outward, and the original suspensorial cartilage (the quadrate) has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part.Lower Jaw--Distinct bones in the frog; one mass in the dog.Otic Capsule--Position as specified.Onecentre of ossification in the frog forming pro-otic;severalfuse together and form periotic of the dog.There is no bulla and no external ear in the frog.Palate--In the frog the posterior nares open into the front of the mouth. In the dog the maxillae and palatines send plates down and in (the palatine plates) to cut off a nasal passage from the rest of the buccal chamber, and carry the posterior nares back to the pharynx, thus cutting the vomers off from the mouth roof.The pterygoids in the dog are much reduced, and do not reach back to the suspensorium.The frog has no lachrymal bone.
Of the frog is a cylindrical box, from which the otic capsules project conspicuously on either side. It contains only two ossifications in its cartilaginous substance (the sphen-ethmoid and the ex-occipital), being protected by the membrane bones, the parieto-frontals above and the parasphenoid below.In the mammal it is enormously inflated, and the otic capsules are imbedded in its wall. There are supra- and basi- as well as ex-occipital bones; the para-sphenoid is (? entirely) gone, and its place is taken by the basi- and pre-sphenoids, and the lateral walls contain fresh paired ossifications, the ali- and orbito-sphenoids-- all cartilage bones. The sphenethmoid is perhaps represented in part by the ethmoid.As a result of the inflation of the brain-case, the squamosal, which slopes downward and outward in the frog, and overlies the cartilaginous suspensorium (quadrate cartilage), has become a constituent of the brain-case wall, and slopes downwardly and in.
The point of attachment of the jaw has shifted outward, and the original suspensorial cartilage (the quadrate) has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part.
Distinct bones in the frog; one mass in the dog.
Position as specified.Onecentre of ossification in the frog forming pro-otic;severalfuse together and form periotic of the dog.There is no bulla and no external ear in the frog.
In the frog the posterior nares open into the front of the mouth. In the dog the maxillae and palatines send plates down and in (the palatine plates) to cut off a nasal passage from the rest of the buccal chamber, and carry the posterior nares back to the pharynx, thus cutting the vomers off from the mouth roof.The pterygoids in the dog are much reduced, and do not reach back to the suspensorium.The frog has no lachrymal bone.