[119]It is not difficult to picture a possible derivation of delamination from invagination; while a comparison of the formation of the inner layers (mesoblast and hypoblast) in Ascetta (amongst the Sponges), and in the Echinodermata, shews a very simple way in which it is possible to conceive of a passage of delamination into invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are budded off from the inner wall of the blastosphere, especially at one point; while in Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the hypoblast, while from the walls of this sack amœboid cells are budded off which give rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off at one point in Ascetta gradually to form an invaginated sack, while the mesoblast cells continued to be budded off as before, we should pass from the delaminate type of Ascetta to the invaginate type of an Echinoderm.[120]The above list is somewhat tentative; and future investigations will probably shew that many of the statements at present current about the position of the blastopore are inaccurate.[121]The forms in which the position of the blastopore in relation to the mouth or anus is not known are marked with an asterisk.[122]J. Parker, “On the Histology ofHydra fusca,”Quart. Journ. Micr. Science,vol.XX.1880; and El. Metschnikoff,“Ueb. die intracelluläre Verdauung bei Cœlenteraten,”Zoologischer Anzeiger,No. 56, vol.III.1880 and Lankester, “On the intracellular digestion and endoderm of Limnocodium,”Quart. Journ. Micr. Science,vol.XXI.1881.[123]Vol.II. p.149.[124]The Hertwigs (No.270) have for instance shewn that nervous structures are developed in the hypoblast in the Actinozoa and other Cœlenterata.[125]There is considerable confusion in the use of the names for the embryonic layers. In some cases various tissues formed by differentiations of the primary layers have been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out the inconvenience of this nomenclature. In the case of the Cœlenterata it is difficult to decide in certain instances (e.g.Sympodium) whether the cells which give rise to a particular tissue of the adult are to be regarded as forming a mesoblast,i.e.a middle undifferentiated layer of cells, or whether they arise as already histologically differentiated elements from one of the primary layers. The attempt to distinguish by a special nomenclature the epiblast and hypoblast after and before the separation of the mesoblast, which has been made by Allen Thomson (No.1), appears incapable of being consistently applied, though it is convenient to distinguish a primary and a secondary hypoblast. A proposal of the Hertwigs to adopt special names for the outer and inner limiting membranes of the adult, and for the interposed mass of organs, appears to me unnecessary.[126]The causes which give rise to a retardation of histological differentiation will be dealt with in the second part of this chapter which deals with larval characters and larval forms.[127]The connective-tissue test of the Tunicata, though derived from the epiblast, is not really an example of such a differentiation.[128]M. L. Ranvier. “Sur la structure des glandes sudoripares.”Comptes Rendus,Dec.29, 1879.[129]A. Götte, “Vergleich. Entwick. d. Comatula mediterranea.”Archiv f. mikr. Anat.vol.XII. p.597.[130]The Hertwigs hold that there is a distinct part of the nervous system which was at first differentiated in the mesoblast in many types, amongst others the Mollusca. The evidence in favour of this view is extremely scanty and the view itself appears to me highly improbable.[131]The reader is referred for this subject to the valuable memoirs which have been recently published by the Hertwigs, especially toNo.270. He will find a general account of the subject written before the appearance of the Hertwigs’ memoir inpp.180-182 of VolumeII. of this treatise.[132]It would be interesting to know the history of the various nervous structures found in the walls of the alimentary tract in the higher forms. I have shewn (Development of Elasmobranch Fishes,p.172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over the development of Auerbach’s plexus.[133]The wide occurrence of this process was first pointed out by Rabl. He holds, however, a peculiar modification of the gastræa theory, for which I must refer the reader to his paper (No.284); according to this theory the mesoblast has sprung from a zone of cells of the blastosphere, at the junction between the cells which will be invaginated and the epiblast cells. In the bilateral blastosphere, from which he holds that all the higher forms (Bilateralia) have originated, these cells had a bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. The origin of the mesoblast from the lips of the blastopore is explained by the position of its mother-cells in the blastosphere. It need scarcely be said that the views already put forward as to the probable mode of origin of the mesoblast, founded on the analogy of the Cœlenterata, are quite incompatible with Rabl’s theories.[134]Zoologischer Anzeiger,No.52,p.140. This form has been named by KowalevskyCœloplana Metschnikowii. Kowalevsky’s description appears, however, to be quite compatible with the view that this form is a creeping Ctenophor, in no way related to the Turbellarians.[135]For numerous instances of this kind,videChapterXI. of Vol.III.[136]It has long been known that land and freshwater forms develop without a metamorphosis much more frequently than marine forms. This is probably to be explained by the fact that there is not the same possibility of a land or freshwater species extending itself over a wide area by the agency of free larvæ, and there is, therefore, much less advantage in the existence of such larvæ; while the fact of such larvæ being more liable to be preyed upon than eggs, which are either concealed, or carried about by the parent, might render a larval stage absolutely disadvantageous.[137]The phosphorescence of many larvæ is very peculiar. I should have anticipated that phosphorescence would have rendered them much more liable to be captured by the forms which feed upon them; and it is difficult to see of what advantage it can be to them.[138]The larva of the Brachiopoda does not possess most of the characters mentioned below. It is probably, all the same, a highly differentiated larval form belonging to this group.[139]There is some uncertainty as to the development of the œsophagus in the Echinodermata, but recent researches appear to indicate that it is developed from the hypoblast.[140]For a discussion as to the structure of the Polyzoon larva,videVol.II.p.305.[141]VideVol.II.pp.179 and 191. In this connection attention may be called toCœloplana Metschnikowii, a form described by Kowalevsky,Zoologischer Anzeiger,No.52,p.140, as being intermediate between the Ctenophora and the Turbellaria. As already mentioned, there does not appear to me to be sufficient evidence to prove that this form is not merely a creeping Ctenophor.[142]Quart. Journ. of Micr. Science,Vol.XVII.pp.422-3.[143]VideHubrecht,“Zur Anat. und Phys. d. Nerven-System. d. Nemertinen,”Kön. Akad. Wiss.,Amsterdam; and “Researches on the Nervous System of Nemertines,”Quart. Journ. of Micr. Science, 1880.[144]VideF. M. Balfour, “On some points in the Anat. of Peripatus capensis,”Quart. Journ. of Micr. Science,Vol.XIX. 1879.[145]VideVol.II.p.204.[146]The independent development of the supraœsophageal ganglion and ventral nerve-cord in Chætopoda (videKleinenberg,Development of Lumbricus trapezoides) agrees very satisfactorily with this view.[147]It is quite possible that Phoronis is in no way related to the other Gephyrea.[148]This important memoir only came into my hands after this chapter was already in type.
[119]It is not difficult to picture a possible derivation of delamination from invagination; while a comparison of the formation of the inner layers (mesoblast and hypoblast) in Ascetta (amongst the Sponges), and in the Echinodermata, shews a very simple way in which it is possible to conceive of a passage of delamination into invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are budded off from the inner wall of the blastosphere, especially at one point; while in Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the hypoblast, while from the walls of this sack amœboid cells are budded off which give rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off at one point in Ascetta gradually to form an invaginated sack, while the mesoblast cells continued to be budded off as before, we should pass from the delaminate type of Ascetta to the invaginate type of an Echinoderm.
[120]The above list is somewhat tentative; and future investigations will probably shew that many of the statements at present current about the position of the blastopore are inaccurate.
[121]The forms in which the position of the blastopore in relation to the mouth or anus is not known are marked with an asterisk.
[122]J. Parker, “On the Histology ofHydra fusca,”Quart. Journ. Micr. Science,vol.XX.1880; and El. Metschnikoff,“Ueb. die intracelluläre Verdauung bei Cœlenteraten,”Zoologischer Anzeiger,No. 56, vol.III.1880 and Lankester, “On the intracellular digestion and endoderm of Limnocodium,”Quart. Journ. Micr. Science,vol.XXI.1881.
[123]Vol.II. p.149.
[124]The Hertwigs (No.270) have for instance shewn that nervous structures are developed in the hypoblast in the Actinozoa and other Cœlenterata.
[125]There is considerable confusion in the use of the names for the embryonic layers. In some cases various tissues formed by differentiations of the primary layers have been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out the inconvenience of this nomenclature. In the case of the Cœlenterata it is difficult to decide in certain instances (e.g.Sympodium) whether the cells which give rise to a particular tissue of the adult are to be regarded as forming a mesoblast,i.e.a middle undifferentiated layer of cells, or whether they arise as already histologically differentiated elements from one of the primary layers. The attempt to distinguish by a special nomenclature the epiblast and hypoblast after and before the separation of the mesoblast, which has been made by Allen Thomson (No.1), appears incapable of being consistently applied, though it is convenient to distinguish a primary and a secondary hypoblast. A proposal of the Hertwigs to adopt special names for the outer and inner limiting membranes of the adult, and for the interposed mass of organs, appears to me unnecessary.
[126]The causes which give rise to a retardation of histological differentiation will be dealt with in the second part of this chapter which deals with larval characters and larval forms.
[127]The connective-tissue test of the Tunicata, though derived from the epiblast, is not really an example of such a differentiation.
[128]M. L. Ranvier. “Sur la structure des glandes sudoripares.”Comptes Rendus,Dec.29, 1879.
[129]A. Götte, “Vergleich. Entwick. d. Comatula mediterranea.”Archiv f. mikr. Anat.vol.XII. p.597.
[130]The Hertwigs hold that there is a distinct part of the nervous system which was at first differentiated in the mesoblast in many types, amongst others the Mollusca. The evidence in favour of this view is extremely scanty and the view itself appears to me highly improbable.
[131]The reader is referred for this subject to the valuable memoirs which have been recently published by the Hertwigs, especially toNo.270. He will find a general account of the subject written before the appearance of the Hertwigs’ memoir inpp.180-182 of VolumeII. of this treatise.
[132]It would be interesting to know the history of the various nervous structures found in the walls of the alimentary tract in the higher forms. I have shewn (Development of Elasmobranch Fishes,p.172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over the development of Auerbach’s plexus.
[133]The wide occurrence of this process was first pointed out by Rabl. He holds, however, a peculiar modification of the gastræa theory, for which I must refer the reader to his paper (No.284); according to this theory the mesoblast has sprung from a zone of cells of the blastosphere, at the junction between the cells which will be invaginated and the epiblast cells. In the bilateral blastosphere, from which he holds that all the higher forms (Bilateralia) have originated, these cells had a bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. The origin of the mesoblast from the lips of the blastopore is explained by the position of its mother-cells in the blastosphere. It need scarcely be said that the views already put forward as to the probable mode of origin of the mesoblast, founded on the analogy of the Cœlenterata, are quite incompatible with Rabl’s theories.
[134]Zoologischer Anzeiger,No.52,p.140. This form has been named by KowalevskyCœloplana Metschnikowii. Kowalevsky’s description appears, however, to be quite compatible with the view that this form is a creeping Ctenophor, in no way related to the Turbellarians.
[135]For numerous instances of this kind,videChapterXI. of Vol.III.
[136]It has long been known that land and freshwater forms develop without a metamorphosis much more frequently than marine forms. This is probably to be explained by the fact that there is not the same possibility of a land or freshwater species extending itself over a wide area by the agency of free larvæ, and there is, therefore, much less advantage in the existence of such larvæ; while the fact of such larvæ being more liable to be preyed upon than eggs, which are either concealed, or carried about by the parent, might render a larval stage absolutely disadvantageous.
[137]The phosphorescence of many larvæ is very peculiar. I should have anticipated that phosphorescence would have rendered them much more liable to be captured by the forms which feed upon them; and it is difficult to see of what advantage it can be to them.
[138]The larva of the Brachiopoda does not possess most of the characters mentioned below. It is probably, all the same, a highly differentiated larval form belonging to this group.
[139]There is some uncertainty as to the development of the œsophagus in the Echinodermata, but recent researches appear to indicate that it is developed from the hypoblast.
[140]For a discussion as to the structure of the Polyzoon larva,videVol.II.p.305.
[141]VideVol.II.pp.179 and 191. In this connection attention may be called toCœloplana Metschnikowii, a form described by Kowalevsky,Zoologischer Anzeiger,No.52,p.140, as being intermediate between the Ctenophora and the Turbellaria. As already mentioned, there does not appear to me to be sufficient evidence to prove that this form is not merely a creeping Ctenophor.
[142]Quart. Journ. of Micr. Science,Vol.XVII.pp.422-3.
[143]VideHubrecht,“Zur Anat. und Phys. d. Nerven-System. d. Nemertinen,”Kön. Akad. Wiss.,Amsterdam; and “Researches on the Nervous System of Nemertines,”Quart. Journ. of Micr. Science, 1880.
[144]VideF. M. Balfour, “On some points in the Anat. of Peripatus capensis,”Quart. Journ. of Micr. Science,Vol.XIX. 1879.
[145]VideVol.II.p.204.
[146]The independent development of the supraœsophageal ganglion and ventral nerve-cord in Chætopoda (videKleinenberg,Development of Lumbricus trapezoides) agrees very satisfactorily with this view.
[147]It is quite possible that Phoronis is in no way related to the other Gephyrea.
[148]This important memoir only came into my hands after this chapter was already in type.
Introduction.
Our knowledge of the development of the organs in most of the Invertebrate groups is so meagre that it would not be profitable to attempt to treat systematically the organogeny of the whole animal kingdom.
For this reason the plan adopted in this section of the work has been to treat somewhat fully the organogeny of the Chordata, which is comparatively well known; and merely to indicate a few salient facts with reference to the organogeny of other groups. In the case of the nervous system, and of some other organs which especially lend themselves to this treatment, such as the organs of special sense and the excretory system, a wider view of the subject has been taken; and certain general principles underlying the development of other organs have also been noticed.
The classification of the organs is a matter of some difficulty. Considering the character of this treatise it seemed desirable to arrange the organs according to the layers from which they are developed. The compound nature of many organs,e.g.the eye and ear, renders it, however, impossible to carry out consistently such a mode of treatment. I have accordingly adopted a rough classification of the organs according to the layers, dropping the principle where convenient, as, for instance, in the case of the stomodæum and proctodæum.
The organs which may be regarded as mainly derived fromthe epiblast are (1) the skin; (2) the nervous system; (3) the organs of special sense.
Those from the mesoblast are (1) the general connective tissue and skeleton; (2) the vascular system and body cavity; (3) the muscular system; (4) the urinogenital system.
Those from the hypoblast are the alimentary tract and its derivates; with which the stomodæum and proctodæum and their respective derivates are also dealt with.
Bibliography.
General works dealing with the development of the organs of the Chordata.
(291)K. E. von Baer.Ueber Entwicklungsgeschichte d. Thiere.Königsberg, 1828-1837.(292)F. M. Balfour.A Monograph on the development of Elasmobranch Fishes.London, 1878.(293)Th. C. W. Bischoff.Entwicklungsgesch. d. Säugethiere U. d. Menschen.Leipzig, 1842.(294)C. Gegenbaur.Grundriss d. vergleichenden Anatomie.Leipzig, 1878.Videalso English translation,Elements of Comp. Anatomy. London, 1878.(295)M. FosterandF. M. Balfour.The Elements of Embryology.Part I. London, 1874.(296)Alex. Götte.Entwicklungsgeschichte d. Unke.Leipzig, 1875.(297)W. His.Untersuch. üb. d. erste Anlage d. Wirbelthierleibes.Leipzig, 1868.(298)A. Kölliker.Entwicklungsgeschichte d. Menschen u. der höheren Thiere.Leipzig, 1879.(299)H. Rathke.Abhandlungen ü. Bildung und Entwicklungsgeschichte d. Menschen u. d. Thiere.Leipzig, 1838.(300)H. Rathke.Entwicklungs. d. Natter.Königsberg, 1839.(301)H. Rathke.Entwicklungs. d. Wirbelthiere.Leipzig, 1861.(302)R. Remak.Untersuchungen üb. d. Entwicklung d. Wirbelthiere.Berlin, 1850-1855.(303)S. L. Schenk.Lehrbuch d. vergleich. Embryologie d. Wirbelthiere.Wien, 1874.
In many of the Cœlenterata the outermost layer of the blastoderm is converted as a whole into the skin or ectoderm. The cells composing it become no doubt in part differentiated into muscular elements and in part into nervous elements,&c.; but still it may remain through life as a simple external membrane. This membrane contains in itself indefinite potentialities for developing into various organs, and in all the true Triploblastica these potentialities are more or less realized. The embryonic epiblast ceases in fact, in the higher forms, to become converted as a whole into the epidermis, but first gives rise to parts of the nervous system, organs of special sense, and other parts.
After the formation of these parts the remnant of the epiblast gives rise to the epidermis, and often unites more or less intimately with a subjacent layer of mesoblast, known as the dermis, to form with it the skin.
Various differentiations may arise in the epidermis forming protective or skeletal structures, terminal sense organs, or glands. The structure of the epidermis itself varies greatly, and for Vertebrates its general modifications have been already sufficiently dealt with in chapterXII. Of its special differentiations those of a protective or skeletal nature and those of a glandular nature may be considered in this place.
Protective epidermal structures. These structures constitute a general cuticle or an exoskeleton of scales, hairs, feathers, nails, hoofs,&c.They may be entirely formed fromthe epidermis either as (1) a cuticular deposit, or as (2) a chitinization, a cornification, or calcification of its constituent cells. These two processes run into each other, and are in many cases not easily distinguished. The protective structures of the epidermis may be divided into two groups according as they are formed on theouteror theinnerside of the epidermis. Dermal skeletal structures are in many cases added to them. Amongst the Invertebrata the most widely distributed type of exoskeleton is a cuticle formed on the outer surface of the epidermis, which reaches its highest development in the Arthropoda. In the same class with this cuticle must be placed the molluscan and brachiopod shells, which are developed as cuticular plates on special regions of the epidermis. They differ, however, from the more usual form of cuticle in their slighter adhesion to the subjacent epidermis, and in their more complicated structure. The test of Ascidians is an abnormal form of exoskeleton belonging to this type. It is originally formed (Hertwig and Semper) as a cuticle on the surface of the epidermis; but subsequently epidermic cells migrate into it, and it then constitutes a tissue similar to connective tissue, but differing from ordinary epidermic cuticles in that the cells which deposit it do so over their whole surface, instead of one surface, as is usually the case with epithelial cells.
In the Vertebrata the two types of exoskeleton mentioned above are both found, but that developed on the inner surface of the epidermis is always associated with a dermal skeleton, and that on the outer side frequently so. The type of exoskeleton developed on theinner sideof the general epidermis is confined to the Pisces, where it appears as thescales; but a primitive form of these structures persists as the teeth in the Amphibia and Amniota. The type developed on theouterside of the epidermis is almost entirely[149]confined to the Amphibia and Amniota, where it appears as scales, feathers, hairs, claws, nails,&c.For the histological details as to the formation of these various organs I must refer the reader to treatises on histology, confining my attention here to the general embryological processes which take place in their development.
The most primitive form of the first type of dermal structures is that of the placoid scales of Elasmobranchii[150]. These consist, when fully formed, of a plate bearing a spinous projection. They are constituted of an outer enamel layer on the projecting part, developed as a cuticular deposit of the epidermis (epiblast), and an underlying basis of dentine (the lower part of which may be osseous) with a vascular pulp in its axis. The development (fig. 235) is as follows (Hertwig,No.306). A papilla of the dermis makes its appearance, the outer layer of which gradually calcifies to form the dentine and osseous tissue. This papilla is covered by the columnar mucous layer of the epidermis (e), from which it is separated by a basement membrane, itself a product of the epidermis. This membrane gradually thickens and calcifies, and so gives rise to the enamel cap (o). The spinous point gradually forces its way through the epidermis, so as to project freely at the surface.
The scales of other forms of fishes are to be derived from those of Elasmobranchii. The great dermal plates of many fishes have been formed by the concrescence of groups of such scales. The dentine in many cases partially or completely atrophies, leaving the major part of the scale formed of osseous tissue; such plates often become parts of the internal skeleton.
Illustration: Figure 235Fig. 235. Vertical section through the skin of an embryonic Shark, to shew a developing placoid scale.(From Gegenbaur; after O. Hertwig.)E.epidermis;C.layers of dermis;d.uppermost layer of dermis;p.papilla of dermis;e.mucous layer of epidermis;o.enamel layer.
Fig. 235. Vertical section through the skin of an embryonic Shark, to shew a developing placoid scale.(From Gegenbaur; after O. Hertwig.)E.epidermis;C.layers of dermis;d.uppermost layer of dermis;p.papilla of dermis;e.mucous layer of epidermis;o.enamel layer.
The teeth, as will be more particularly described in the section on the alimentary tract, are formed by a modification of the same process as the placoid scales, in which a ridge of the epithelium grows inwards to meet a connective tissue papilla, so that the development of the teeth takes place entirely below the superficial layer of epidermis.
In most Teleostei the enamel and dentine layers have disappeared, and the scales are entirely formed of a peculiar calcified tissue developed in the dermis.
The cuticle covering the scales of Reptiles is the simplest type of protective structure formed on the outer surface of the epidermis. The scales consist of papillæ of the dermis and epidermis; and are covered by a thickened portion of a two-layered cuticle, formed over the whole surface of the body from a cornification of the superficial part of the epidermis. Dermal osseous plates may be formed in connection with these scales, but are never of course united with the superficial cuticle.
Feathers are probably special modifications of such scales. They arise from an induration of the epidermis of papillæ containing a vascular core. The provisional down, usually present at the time of hatching, is formed by the cornification of longitudinal ridges of the mucous layer of the epidermis of the papillæ; each cornified ridge giving rise to a barb of the feather. The horny layer of the epidermis forms a provisional sheath for the developing feather below. When the barbs are fully formed this sheath is thrown off, the vascular core dries up, and the barbs become free except at their base.
Without entering into the somewhat complicated details of the formation of the permanent feathers, it may be mentioned that the calamus or quill is formed by a cornification in the form of a tube of both layers of the epidermis at the base of the papilla. The quill is open at both ends, and to it is attached the vexillum or plume of the feather. In a typical feather this is formed at the apex of the papilla from ridge-like thickenings of the mucous layer of the epidermis, arranged in the form of a longitudinal axis, continuous with the cornified mucous layer of the quill, and from lateral ridges. These subsequently become converted into the axis and barbs of the plume. The external epidermic layer becomes converted into a provisional horny sheath for the true feather beneath.
On the completion of the plume of the feather the external sheath is thrown off, leaving it quite free, and the vascular core belonging to it shrivels up. The papilla in which the feather is formed becomes at a very early period secondarily enveloped in a pit or follicle which gradually deepens as the development of the feather is continued.
Hairs (Kölliker,No.298) are formed in solid processes of the mucous layer of the epidermis, which project intothe subjacent dermis. The hair itself arises from a cornification of the cells of the axis of one of the above processes; and is invested by a sheath similarly formed from the more superficial epidermic cells. A small papilla of the dermis grows into the inner end of the epidermic process when the hair is first formed. The first trace of the hair appears close to this papilla, but soon increases in length, and when the end of the hair projects from the surface, the original solid process of the epidermis becomes converted into an open pit, the lumen of which is filled by the root of the hair. Hairs differ in their mode of formation from scales in a manner analogous to that in which the teeth differ from ordinary placoid scales;i.e.they are formed in inwardly directed projections of the epidermis instead of upon free papillæ at the surface.
Nails (Kölliker,No.298) are developed on special regions of the epidermis, known as the primitive nail beds. They are formed by the cornification of a layer of cells which makes its appearance between the horny and mucous layers of the epidermis. The distal border of the nail soon becomes free, and the further growth is effected by additions to the under side and attached extremity of the nail.
Although the nail at first arises in the interior of the epidermis, yet its position on the outer side of the mucous layer clearly indicates with which group of epidermic structures it should be classified.
Dermal skeletal structures. We have seen that in the Chordata skeletal structures, which were primitively formed of both an epidermic and dermic element, may lose the former element and be entirely developed in the dermis. Amongst the Invertebrata there are certain dermal skeletal structures which are evolved wholly independently of the epidermis. The most important of these structures are the skeletal plates of the Echinodermata.
Glands. The secretory part of the various glandular structures belonging to the skin is invariably formed from the epidermis. In Mammalia it appears that these glands are always formed as solid ingrowths of the mucous layer (Kölliker,No.298). The ends of these ingrowths dilate to form the true glandular part of the organs, while the stalks connecting the glandular portions with the surface form the ducts. In the case of the sweat-glands the lumen of the duct becomes first established. Its formation is inaugurated by the appearance ofthe cuticle, and appears first at the inner end of the duct and thence extends outwards (Ranvier,No.311). In the sebaceous glands the first secretion is formed by a fatty modification of the whole of the central cells of the gland.
The muscular layer of the secreting part of the sweat-glands is formed, according to Ranvier (No.311), from a modification of the deeper layer of the epidermic cells.
The Mammary Glandsarise in essentially the same manner as the other glands of the skin[151]. The glands of each side are formed as a solid bud of the mucous layer of the epidermis. From this bud processes sprout out, each of which gives rise to one of the numerous glands of which the whole organ is formed. Two very distinct types in the relation of the ducts of the glands to the nipple are found (Gegenbaur,No.313).
Bibliography of Epidermis.
General.
(304)T. H. Huxley. “Tegumentary organs.” Todd’sCyclopædia of Anat. and Physiol.(305)P. Z. Unna.“Histol. u. Entwick. d. Oberhaut.”Archiv f. mikr. Anat.Vol.XV.1876.VidealsoKölliker(No.298).
Scales of the Pisces.
(306)O. Hertwig. “Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zähne d. Selachier.”Jenaische Zeitschrift,Vol.VIII. 1874.(307)O. Hertwig. “Ueber d. Hautskelet d. Fische.”Morphol. Jahrbuch,Vol.II. 1876. (Siluroiden u. Acipenseridæ.)(308)O. Hertwig. “Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus).”Morph. Jahrbuch,Vol.V. 1879.
Feathers.
(309)Th. Studer.Die Entwick. d. Federn.Inaug. Diss. Bern, 1873.(310)Th. Studer. “Beiträge z. Entwick. d. Feder.”Zeit. f. wiss. Zool.,Vol.XXX. 1878.
Sweat-glands.
(311)M. S. Ranvier. “Sur la structure des glandes sudoripares.”Comptes Rendus,Dec.29, 1879.
Mammary glands.
(312)C. Creighton. “On the development of the Mamma and the Mammary function.”Jour. of Anat. and Phys.,Vol.XI. 1877.(313)C. Gegenbaur. “Bemerkungen üb. d. Milchdrüsen-Papillen d. Säugethiere.”Jenaische Zeit.,Vol.VII. 1873.(314)M. Huss. “Beitr. z. Entwick. d. Milchdrüsen b. Menschen u. b. Wiederkäuern.”Jenaische Zeit.,Vol.VII. 1873.(315)C. Langer. “Ueber d. Bau u. d. Entwicklung d. Milchdrüsen.”Denk. d. k. Akad. Wiss. Wien,Vol.III. 1851.
[149]The horny teeth of the Cyclostomata are structures belonging to this group.[150]For the most important contributions on this subject from which the facts and views here expressed are largely derived,videO. Hertwig,Nos.306-308.[151]For a very different view on this subjectvideCreighton (No.312).
[149]The horny teeth of the Cyclostomata are structures belonging to this group.
[150]For the most important contributions on this subject from which the facts and views here expressed are largely derived,videO. Hertwig,Nos.306-308.
[151]For a very different view on this subjectvideCreighton (No.312).
Origin of the Nervous System.
One of the most important recent embryological discoveries is the fact that the central nervous system, in all the Metazoa in which it is fully established, is (with a few doubtful exceptions) derived from the primitive epiblast[152]. As we have already seen that the epiblast represents to a large extent the primitive epidermis, the fact of the nervous system being derived from the epiblast implies that the functions of the central nervous system, which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by step removed from the surface, and has finally become in the higher types a well-defined organ imbedded in the subdermal tissues.
Before considering in detail the comparative development of the nervous system, it will be convenient shortly to review the present state of our knowledge on the general process of its evolution.
This process may be studied either embryologically, or by a comparison of the various stages in its evolution preserved in living forms. Both the methods have led to important results.
The embryological evidence shews that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body, while the central nervous system itself has arisen from the concentration of such cells in special tracts. In the Chordata at any rate the nerves arise as outgrowths of the central organ.
Another important fact shewn by embryology is that the central nervous system, and percipient portions of the organs of special sense, especially of optic organs, are often formed from the same part of the primitive epidermis. Thus the retina of the Vertebrate eye is formed from the two lateral lobes of the primitive fore-brain.
The same is true for the compound eyes of some Crustacea. The supraœsophageal ganglia of these animals are formed in the embryo from two thickened patches of the epiblast of the procephalic lobes. These thickened patches become gradually detached from the surface, remaining covered by a layer of epidermis. They then constitute the supraœsophageal ganglia; but they form not only the ganglia, but also the retinulæ of the eye—the parts in fact which correspond to the rods and cones in our own retina. The accessory parts of these organs of special sense,viz.the crystalline lens of the Vertebrate eye, and the corneal lenses and crystalline cones of the Crustacean eye, are independently formed from the epiblast after the separation of the part which becomes the central nervous system.
In the Acraspedote Medusæ the rudimentary central nervous system has the form of isolated rings, composed of sense-cells prolonged into nervous fibres, surrounding the stalks of tentacle-like organs, at the ends of which are placed the sense-organs.
This close connection between certain organs of special sense and ganglia is probably to be explained by supposing that the two sets of structures actually originatedpari passu.
We may picture the process as being somewhat as follows:—
It is probable that in simple ancestral organisms the whole body was sensitive to light, but that with the appearance of pigment-cells in certain parts of the body, the sensitiveness to light became localised to the areas where the pigment-cells were present. Since, however, it was necessary that stimuli received by such organs should be communicated to other partsof the body, some of the epidermic cells in the neighbourhood of the pigment-spots, which were at first only sensitive in the same manner as other cells of the epidermis, became gradually differentiated into special nerve-cells. As to the details of this differentiation embryology does not as yet throw any great light; but from the study of comparative anatomy there are grounds for thinking that it was somewhat as follows:—Cells placed on the surface sent protoplasmic processes of a nervous nature inwards, which came into connection with nervous processes from similar cells placed in other parts of the body. The cells with such processes then became removed from the surface, forming a deeper layer of the epidermis below the sensitive cells of the organ of vision. With the latter cells they remained connected by protoplasmic filaments, and thus they came to form a thickening of the epidermis underneath the organ of vision, the cells of which received their stimuli from those of the organ of vision, and transmitted the stimuli so received to other parts of the body. Such a thickening would obviously be the rudiment of a central nervous system, and is in fact very similar to the rudimentary ganglia of the Acraspeda mentioned above. It is easy to see by what steps it might become larger and more important, and might gradually travel inwards, remaining connected with the sense-organ at the surface by protoplasmic filaments, which would then constitute nerves. The rudimentary eye would at first merely consist of cells sensitive to light, and of ganglion-cells connected with them; while at a later period optical structures, constituting a lens capable of throwing an image of external objects upon it, would be developed, and so convert the whole structure into a true organ of vision. It has thus come about that, in the development of the individual, the retina is often first formed in connection with the central nervous system, while the lenses of the eye are independently evolved from the epidermis at a later period.
A series of forms of the Cœlenterata and Platyelminthes affords us examples of various stages in the differentiation of a central nervous system[153].
In sea-anemones (Hertwigs,No.321) there are, for instance, no organs of special sense, and no definite central nervous system. There are, however, scattered throughout the skin, and also throughout the lining of the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with a long hair, and are prolonged on their inner side into fine processes which penetrate into the deeper part of the epithelial layer of the skin or digestive wall. They eventually join a fine network of protoplasmic fibres which forms a special layer immediately within the epithelium. The fibres of this network are no doubt essentially nervous. In addition to fibres there are,moreover, present in the network cells of the same character as the multipolar ganglion-cells in the nervous system of Vertebrates, and some of these cells are characterised by sending a process into the superjacent epithelium. Such cells are obviously intermediate between neuro-epithelial cells and ganglion-cells; and it is probable that the nerve-cells are, in fact, sense-cells which have travelled inwards and lost their epithelial character.