Book - The Elements of Embryology - Mammalian 3
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The Development of the Organs in Mammalia
IN chap, X. we have described the early stages and general development of the mammalian embryo. In the present chapter we propose to examine the formation of such mammalian organs as differ in their development from those of the chick. This will not be a work of any considerable extent, as in all essential points the development of the organs in the two groups is the same. They will be classified according to the germinal layers from which they originate.
The Organs Derived from the Epiblast
Hairs are formed in solid processes of the deep (Malpighian) layer of the epidermis, which project into the 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.
The development of nails has been already described on p. 283.
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 Malpighian layer. 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 of the cuticle, and appears first at the inner end of the duct and thence extends outwards. 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 said to be formed from a modification of the deeper layer of the epidermic cells.
The mammary glands arise in essentially the same manner as the other glands of the skin. The glands of each side are formed as a solid bud of the Malpighian 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.
The central nervous system
The development of the spinal cord in Mammals differs in no important respects from that of the chick, and we have nothing to add to the account we have already given of its general development and histogenesis in that animal. The development of the brain however will be described at greater length, and some additional facts relative to the development of the Avian brain will be mentioned.
The first differentiation of the brain takes place in Mammalia before the closure of the medullary folds, and results as in the chick in the formation of the three cerebral vesicles, the fore-, mid- and hind-brain (Fig. 106, B). A cranial flexure precisely resembling that of the chick soon makes its appearance.
The hind brain early becomes divided into two regions, the rudimentary medulla oblongata and cerebellum.
The posterior section, the medulla, undergoes changes of a somewhat complicated character. In the first place its roof becomes very much extended and thinned out. At the raphe, where the two lateral halves of the brain originally united, a separation, as it were, takes place, and the two sides of the brain become pushed apart, remaining united by only a very thin layer of nervous matter, consisting of a single row of flattened cells (Fig. 40). As a result of this peculiar growth in the brain, the roots of the nerves of the two sides, which were originally in contact at the dorsal summit of the brain, become carried away from one another, and appear to rise at the sides of the brain.
The thin roof of the fourth ventricle thus formed is somewhat rhomboidal in shape.
At a later period the blood-vessels of the pia mater form a rich plexus over the anterior part of this thin roof which becomes at the same time somewhat folded. The whole structure is known as the tela vasculosa or choroid plexus of the fourth ventricle (Fig. 119, chd 4). The floor of the whole hind -brain becomes thickened, and there very soon appears on its outer surface a layer of longitudinal non-medullated nerve-fibres, similar to those which first appear on the spinal cord (p. 252). They are continuous with a similar layer of fibres on the floor of the mid-brain, where they constitute the crura cerebri. On the ventral floor of the fourth ventricle is a shallow continuation of the anterior fissure of the spinal cord.
Subsequently to the longitudinal fibres already spoken of, there develope first the olivary bodies of the ventral side of the medulla, and at a still later period the pyramids. The fasciculi teretes in the cavity of the fourth ventricle are developed shortly before the pyramids.
When the hind-brain becomes divided into two regions the roof of the anterior part does not become thinned out like that of the posterior, but on the contrary, becomes somewhat thickened and forms a bandlike structure roofing over the anterior part of thc k fourth ventricle (Fig. 39 c6).
This is a rudiment of the cerebellum, and in all Craniate Vertebrates it at first presents this simple structure and insignificant size.
In Birds the cerebellum attains a very considerable development (Fig. 119 cbl), consisting of a folded central lobe with an arbor vitse, into which the fourth ventricle is prolonged. There are two small lateral lobes, apparently equivalent to the flocculi.
In Mammalia the cerebellum attains a still greater development. The median lobe or vermiform process is first developed. In the higher Mammalia the lateral parts constituting the hemispheres of the cerebellum become formed as swellings at the sides at a considerably later period; these are hardly developed in the Monotremata and Marsupialia.
|Fig. 119. Longitudinal section through the brain of a chick of ten days. (After Mihalkovics.)
hms. cerebral hemispheres ; alf. olfactory lobe ; alf1 olfactory nerve ; ggt. corpus striatum ; oma. anterior commissure ; did 3. choroid plexus of the third ventricle ; pin. pineal gland ; cmp. posterior commissure ; trm. lamina terminalis ; chm. optic chiasma ; inf. infundibulum ; hph. pituitary body ; bgm. commissure of Sylvius (roof of iter a tertio ad quartum ventriculum) ; vma. velum medullse anterius (valve of Vieussens) ; cbl. cerebellum ; chd 4. choroid plexus of the fourth ventricle ; obt 4. roof of fourth ventricle ; obi. medulla oblongata ; pns. commissural part of medulla ; inv. sheath of brain ; bis. basilar artery ; crts. internal carotid.
The cerebellum is connected with the roof of the mid-brain in front and with the choroid plexus of the fourth ventricle behind by delicate membranous structures, known as the velum medullse anterius (valve of Yieussens) (Fig. 119 vma) and the velum medullse posterius.
The pons Varolii is formed on the ventral side of the floor of the cerebellar region as a bundle of transverse fibres at about the same time as the olivary bodies. It is represented in Birds by a small number of transverse fibres on the floor of the hind-brain immediately below the cerebellum.
The changes undergone by the mid-brain are simpler than those of any other part of the brain. It forms, on the appearance of the cranial flexure, an unpaired vesicle with a vaulted roof and^ curved floor, at the front end of the long axis of the body (Fig. 67, MB). It is at this period in Mammalia as well as in Aves relatively much larger than in the adult: its cavity is known as the iter a tertio ad quartum ventriculum or aqueductus Sylvii.
The roof of the mid-brain is sharply constricted off from the divisions of the brain in front of and behind it, but these constrictions do not extend to the floor.
In Mammalia the roof and sides give rise to two pairs of prominences, the corpora quadrigemina.
These prominences, which are simply thickenings not containing' any prolongations of the iter, become
first visible on the appearance of an oblique transverse furrow, by which the whole mid-brain is divided into an anterior and posterior portion. The anterior portion is further divided by a longitudinal furrow into the two anterior tubercles (nates) ; but it is not until later on that the posterior portion is similarly divided longitudinally into the two posterior tubercles (testes).
The floor of the mid -brain, bounded posteriorly by the pons Varolii, becomes developed and thickened into the crura cerebri. The corpora geniculata interna also belong to this division of the brain.
The early development of the forebrain in Mammals is the same as in the chick. It forms at first a single vesicle without a trace of separate divisions, but very early buds off the optic vesicles, whose history is described with that of the eye. The anterior part becomes prolonged and at the same time somewhat dilated. At first there is no sharp boundary between the primitive fore-brain and its anterior prolongation, but there shortly appears a constriction which passes from above obliquely forwards and downwards.
Of these two divisions the posterior becomes the thalamencephalon, while the anterior and larger division forms the rudiment of the cerebral hemispheres (Fig. 39 cer) and olfactory lobes. For a considerable period this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction dividing it into two lobes.
The thalamencephalon forms at first a simple vesicle, the walls of which are of a nearly uniform thickness and formed of the usual spindle-shaped cells.
The cavity it contains is known as the third ventricle. Anteriorly it opens widely into the cerebral rudiment, and posteriorly into the ventricle of the mid-brain. The* opening into the cerebral rudiment becomes the foramen of Monro.
For convenience of description we may divide the thalamencephalon into three regions, viz. (1) the floor, (2) the sides, and (3) the roof.
The floor becomes divided into two parts: an anterior part, giving origin to the optic nerves, in which is formed the optic chiasma ; and a posterior part, which becomes produced into a prominence at first inconspicuous the rudiment of the infundibulum (Fig. 39 In). This cornes in contact with the involution from the mouth which gives rise to the pituitary body (Fig. 39 pt).
In Birds, although there is a close connection between the pituitary body and the infundibulum, there is no actual fusion of the two. In Mammalia the case is different. The part of the infundibulum which lies at the hinder end of the pituitary body is at first a simple finger-like process of the brain (Fig. 120 inf)\ but its end becomes swollen, and the lumen in this part becomes obliterated. Its cells, originally similar to those of the other parts of the nervous system, and even containing differentiated nerve-fibres, partly atrophy and partly assume an indifferent form, while at the same time there grow in amongst them numerous vascular and connective-tissue elements. The process of the infundibulum thus metamorphosed becomes inseparably connected with the true pituitary body, of which it is usually described as the posterior lobe.
In the later stages of development the unchanged portion of the infundibulum becomes gradually prolonged and forms an elongated diverticulum of the third ventricle, the apex of which is in contact with the pituitary body (Fig. 120 hph).
The posterior part of the primitive infundibulum becomes the corpus albicans, which is double in Man and the higher Apes ; the ventral part of the posterior wall forms the tuber cinereum. Laterally, at the junction of the optic thalami and infundibulum, there are continued some of the fibres of the crura cerebri, which arc probably derived from the walls of the infundibulum.
The sides of the thalamencephalon become very early thickened to form the optic thalami, which constitute the most important section of the thalamencephalon. These are separated on their inner aspect from the infundibular region by a somewhat S-shaped groove, known as the sulcus of Monro, which ends in the foramen of Monro. They also become secondarily united by a transverse commissure, the grey or middle commissure, which passes across the cavity of the third ventricle.
The roof undergoes more complicated changes. It becomes divided, on the appearance of the pineal gland as a sm^all papilliform outgrowth (the development of which is dealt with below), into two regions a longer anterior in front of the pineal gland, and a shorter posterior. The anterior region becomes at an early period excessively thin, and at a later period, when the roof of the thalamencephalon is shortened by the approach of the cerebral hemispheres to the mid-brain, it becomes (vide Fig. 120 did 3) considerably folded, while at the same time a vascular plexus is formed in the pia mater above it. On the accomplishment of these changes it is known as the tela choroidea of the third ventricle.
|Fig. 120. Longitudinal vertical section through the anterior part of the brain of an embryo rabbit of four centimetres. (After Mihalkovics.)
In the roof of the third ventricle behind the pineal gland there appear transverse commissural fibres, forming a structure known as the posterior commissure, which connects together the two optic thalarni.
The most remarkable organ in the roof of the thalamencephalon is the pineal gland, which is developed as a hollow papilliform outgrowth of the roof, and is at first composed of cells similar to those of the other parts of the central nervous system (Fig. 120 pin). It is directed backwards over the hinder portion of the roof of the thalamencephalon.
In Birds (p. 116) the primitive outgrowth to form the pineal gland becomes deeply indented by vascular connective-tissue ingrowths, so that it assumes a dendritic structure (Fig. 119 pin). The proximal extremity attached to the roof of the thalamencephalon soon becomes solid and forms a special section, known as the infra-pineal process. The central lumen of the free part of the gland finally atrophies, but the branches still remain hollow. The infra-pineal process becomes reduced to a narrow stalk, connecting the branched portion of the body with the brain.
In Mammalia the development of the pineal gland is generally similar to that of Birds. The original outgrowth becomes branched, but the follicles or lobes to which the branching gives rise eventually become solid (Fig. 120 pin). An infra-pineal process is developed comparatively late, and is not sharply separated from the roof of the brain.
No satisfactory suggestions have yet been offered as to the nature of the pineal gland. It appears to possess in all forms an epithelial structure, but, except at the base of the stalk (infra-pineal process) in Mammalia, in the wall of which there are nerve-fibres, no nervous structures are present in it in the adult state.
The cerebral hemispheres
It will be convenient to treat separately the development of the cerebral hemispheres proper, and that of the olfactory lobes.
In the cerebral rudiment two parts may be distinguished, viz. the floor and the roof. The former gives rise to the ganglia at the base of the hemispheres, the corpora striata, the latter to the hemispheres proper.
The first change which takes place consists in the roof growing out into two lobes, between which a shallow median constriction makes its appearance (Fig. 121).
Fig. 121 Diagrammatic Longitudinal Horizontal Section through the Fore-Brain
- 3.v. third ventricle ; Iv. lateral ventricle ; It. lamina terminalis ; ce. cerebral hemisphere ; op. th. optic thalamus.
The two lobes thus formed are the rudiments of the two hemispheres. The cavity of each of them opens by a widish aperture into a cavity at the base of the cerebral rudiment, which again opens directly into the cavity of the third ventricle (3 v). The Y-shaped aperture thus formed, which leads from the cerebral hemispheres into the third ventricle, is the foramen of Monro. The cavity (Iv) in each of the rudimentary hemispheres is a lateral ventricle. The part of the cerebrum which lies between the two hemispheres, and passes forwards from the roof of the third ventricle round the end of the brain to the optic chiasma below, is the rudiment of the lamina terminalis (Figs. 121 It and 123 trm). Up to this point the development of the cerebrum is similar in all Vertebrata, and in some forms it practically does not proceed much further.
The cerebral hemispheres undergo in Mammalia the most complicated development. The primitive unpaired cerebral rudiment becomes, as in lower Vertebrates, bilobed, and at the same time divided by the ingrowth of a septum of connective tissue into two distinct hemispheres (Figs. 125 and 124 / and 122 i). From this septum is formed the falx cerebri and other parts.
The hemispheres contain at first very large cavities, communicating by a wide foramen of Monro with the third ventricle (Fig. 124). They grow rapidly in size, and extend, especially backwards, and gradually cover the thalamencephalon and the mid-brain (Fig. 122 i,f). The foramen of Monro becomes very much narrowed and reduced to a mere slit.
The walls are at first nearly uniformly thick, but the floor becomes thickened on each side, and gives rise to the corpus striatum (Figs. 124 and 125 st}. The corpus striatum projects upwards into each lateral ventricle, and gives to this a somewhat semilunar form, the two horns of which constitute the permanent anterior and descending cornua of the lateral ventricles (Fig. 126 st).
Fig. 122. Brain of a Three Months' Human Embryo. Natural size (From Kolliker.)
- 1. From above with the dorsal part of hemispheres and midbrain removed ; 2. From below. f. anterior part of cut wall of the hemisphere ; f'. cornu ammonis ; tho. optic thalamus ; cst. corpus striatum ; to. optic tract ; cm. corpora mammillaria ; p. pons Yarolii.
With the further growth of the hemisphere the corpus striatum loses its primitive relations to the descending cornu. The reduction in size of the foramen of Monro above mentioned is, to a large extent, caused by the growth of the corpora striata.
The corpora striata are united at their posterior border with the optic thalami. In the later stages of development the area of contact between these two pairs of ganglia increases to a large extent (Fig. 125), and the boundary between them becomes somewhat obscure, so that the sharp distinction which exists in the embryo between the thalamencephalon and cerebral hemispheres becomes lost.
|Fig. 123. Transverse section through the brain of a rabbit five centimetres. (After Mihalkovics.)
The section passes through nearly the posterior border of the septum lucidum, immediately in front of the foramen of Monro.
hms. cerebral hemispheres ; cal. corpus callosum ; amm. cornu ammonis (hippocampus major) ; cms. superior commissure of the cornua ammonis ; spt. septum lucidum ;frx2. anterior pillars of the fornix ; cma. anterior commissure ; trm. lamina terminalis ; sir. corpus striatum ; Itf. nucleus lenticularis of corpus striatum ; vtr 1. lateral ventricle ; vtr 3. third ventricle ; ipl. slit between cerebral hemispheres.
The outer wall of the hemispheres gradually thickens, while the inner wall becomes thinner. In the latter, two curved folds, projecting towards the interior of the lateral ventricle, become formed. These folds extend from the foramen of Monro along nearly the whole of what afterwards becomes the descending cornu of the lateral ventricle. The upper fold becomes the hippocampus major (cornu ammonis) (Figs. 123 amm, 124 and 125 h, and 126 am).
The wall of the lower fold becomes very thin, and a vascular plexus, derived from the connective-tissue septum between the hemispheres, and similar to that of the roof of the third ventricle, is formed outside it. It constitutes a fold projecting into the cavity of the lateral ventricle, and together with the vascular connective tissue in it gives rise to the choroid plexus of the lateral ventricle (Figs. 124 and 125 pi).
It is clear from the above description that a marginal fissure leading into the cavity of the lateral ventricle does not exist in the sense often implied in works on human anatomy, since the epithelium covering the choroid plexus, and forming the true wall of the brain, is a continuous membrane. The epithelium of the choroid plexus of the lateral ventricle is quite independent of that of the choroid plexus of the third ventricle, though at the foramen of Monro the roof of the third ventricle is of course continuous with the inner wall of the lateral ventricle (Fig. 124 s). The vascular elements of the two plexuses form however a continuous structure.
The most characteristic parts of the Mammalian cerebrum are the commissures connecting the two hemispheres. These commissures are (1) the anterior commissure, (2) the fornix, and (3) the corpus callosum, the two latter being peculiar to Mammalia.
|Fig. 124. Transverse section through the brain of a sheep's embryo of 27 cm. in length. (From Kolliker.)
The section passes through the level of the foramen of Monro.
st. corpus striatum ; m. foramen of Monro ; t. third ventricle ; pi. choroid plexus of lateral ventricle ; f. falx cerebri ; th. anterior part of optic thalamus ; ch. optic chiasma ; o. optic nerve ; c. fibres of the cerebral peduncles ; h. cornu ammonis ; p. pharynx ; sa. pre- sphenoid bone ; a. orbi tosphenoid bone ; s. points to part of the roof of the brain at the junction between the roof of the third ventricle and the lamina terminalis ; I. lateral ventricle.
By the fusion of the inner walls of the hemispheres in front of the lamina terminalis a solid septum is formed, continuous behind with the lamina terminalis, and below with the corpora striata (Figs. 120 and 123 spt). It is by a series of differentiations within this septum, the greater part of which gives rise to the septum lucidum, that the above commissures originate. In Man there is a closed cavity left in the septum known as the fifth ventricle, which has however no communication with the true ventricles of the brain.
|Fig. 125. Transverse section through the brain of a sheep's embryo of 27 cm. in length. (From Kolliker.)
The section is taken a short distance behind the section represented in Fig. 124, and passes through the posterior part of the hemispheres and the third ventricle.
st. corpus striatum ; ih. optic thalamus ; to. optic tract ; t. ventricle ; d. roof of third ventricle ; c. fibres of cerebr peduncles ; c. divergence of these fibres into the walls of 1 hemispheres ; e. lateral ventricle with choroid plexus pi ; h. cornu ammonis ; f. primitive falx ; am. alisphenoid ; orbito-sphenoid ; sa. presphenoid ; p. pharynx ; mk. Meckel's cartilage.
In this septum there become first formed, below and behind, the transverse fibres of the anterior commissure (Fig. 120 and Fig. 123 cma), while above and behind these the vertical fibres of the fornix are developed (Fig. 120 and Fig. 123 frx 2). The vertical fibres meet above the foramen of Monro, and thence diverge backwards, as the posterior pillars, to lose themselves in the cornu ammonis (Fig. 123 amm). Ventrally they are continued, as the descending or anterior pillars of the fornix, into the corpus albicans, and thence into the optic thalami 1 .
The corpus callosum is not formed till after the anterior commissure and fornix. It arises in the upper part of the septum formed by the fusion of the lateral walls of the hemispheres (Figs. 120 and 123 cal\ and at first only its curved anterior portion the genu 01 rostrum is developed. This portion is alone found in Monotremes and Marsupials. The posterior portion, which is present in all the Monodelphia, is gradually formed as the hemispheres are prolonged further backwards.
1 Recent observations tend to show that the anterior pillars of the fornix end in the corpus albicans ; and that the fibres running from the latter into the optic thalami are independent of the anterior pillars.
Primitively the Mammalian cerebrum, like that of the lower Vertebrata, is quite smooth. In some of the Mammalia, Monotremata, Insectivora, etc., this condition is retained nearly throughout life, while in the majority of Mammalia a more or less complicated system of fissures is developed on the surface. The most important, and first formed, of these is the Sylvian fissure. It arises at the time when the hemispheres, owing to their growth in front of and behind the corpora striata have assumed somewhat the form of a bean. At the root of the hemispheres the hilus of the bean there is formed a shallow depression which constitutes the first trace of the Sylvian fissure. The part of the brain lying in this fissure is known as the island of Reil.
|Fig. 126. Lateral view of the brain of a calf embryo of 5 cm. (After Mihalkovics.)
The outer wall of the hemisphere is removed, so as to give a view of the interior of the left lateral ventricle.
hs. cut wall of hemisphere ; st. corpus striatum ; am. hippocampus major (cornu ammonis) ; d. choroid plexus of lateral ventricle ; fm. foramen of Monro ; op. optic tract ; in. infundibulum ; mb. mid-brain ; cb. cerebellum ; IV. V. roof of fourth ventricle ; ps. pons Varolii, close to which is the fifth nerve with Gasserian ganglion.
The fissures of the cerebrum may be divided into two classes ; (1) the primitive, (2) the secondary fissures. The primitive fissures are the first to appear ; they owe their origin to a folding of the entire wall of the cerebral vesicles. Many of them are transient structures and early disappear. The most important of those which persist are the hippocainpal, the parieto-occipital, the calcarine (in Man and Apes) sulci and the Sylvian fissures. The secondary fissures appear later, and are due to folds which implicate the cortex of the hemispheres only.
The olfactory lobes
The olfactory lobes, or rhinencephala, are secondary outgrowths of the cerebral hemispheres, and contain prolongations of the lateral ventricles, which may however be closed in the adult state ; they arise at a fairly early stage of development from the under and anterior part of the hemispheres (Fig. 127).
The walls of the brain are at first very thin and, like those of the spinal cord, are formed of a number of ranges of spindle-shaped cells. In the floor of the hind- and mid-brain a superficial layer of delicate nerve-fibres is formed at an early period. This layer appears at first on the floor and sides of the hind-brain, and almost immediately afterwards on the floor and the sides of the mid-brain. The cells internal to the nerve-fibres become differentiated into an innermost epithelial layer lining the cavities of the ventricles, and an outer layer of grey matter.
The similarity of the primitive arrangement and histological characters of the parts of the brain behind the cerebral hemispheres to those of the spinal cord is very conclusively shewn by the examination of any good series of sections. In both brain and spinal cord the white matter forms a cap on the ventral and lateral parts some considerable time before it extends to the dorsal surface. In the medulla oblongata the white matter does not eventually extend to the roof owing to the peculiar degeneration which that part undergoes.
|Fig. 127. Section through the brain and olfactory organ of an embryo of scyllium.
(scyllium = shark)
ch. cerebral hemispheres ; ol.v. olfactory vesicle ; off. olfactory pit ; Sch. Schneiderian folds ; I. olfactory nerve (the reference line has been accidentally carried through the nerve so as to appear to indicate the brain) ; pn. anterior prolongation of pineal gland.
In the case of the fore-brain the walls of the hemispheres become first divided (Kolliker) into a superficial thinner layer of rounded elements, and a deeper and thicker epithelial layer, and between these the fibres of the crura cerebri soon interpose themselves. At a slightly later period a thin superficial layer of white matter, homologous with that of the remainder of the brain, becomes established.
The inner layer, together with the fibres from the crura cerebri, gives rise to the major part of the white matter of the hemispheres and to the epithelium lining the lateral ventricles.
The outer layer of rounded cells becomes divided into (1) a superficial part with comparatively few cells, which, together with its coating of white matter, forms the outer part of the grey matter, and (2) a deeper layer with numerous cells, which forms the main mass of the grey matter of the cortex.
The development of the Mammalian eye is essentially similar to that of the chick (ch. vi.) There are however two features in its development which deserve mention. These are (1) the immense foetal development of the blood-vessels of the vitreous humour and the presence in the embryo of a vascular membrane surrounding the lens, known as the membrana capsulo-pupillaris, (2) the absence of any structure comparable to the pecten, and the presence of the arteria centralis retinae.
In the invagination of the lens (rabbit) a thin layer of mesoblast is carried before it, and is thus transported into the cavity of the vitreous humour. In the folding in of the optic vesicle which accompanies the formation of the lens the optic nerve is included, and on the development of the cavity of the vitreous humour an artery, running in the fold of the optic nerve, passes through the choroid slit into the cavity of the vitreous humour (Fig. 128 acr). The sides of the optic nerve subsequently bend over, and completely envelope this artery, which then gives off branches to the retina, and becomes known as the arteria centralis retince. It is homologous with the arterial limb of the vascular loop projecting into the vitreous humour in Birds.
|Fig. 128. Section through the Eye of a Rabbit Embryo of about Twelve Days
c. epithelium of cornea : 1. lens ; mec. mesoblast growing in from the side to form the cornea ; rt. retina ; a.c.r. arteria centralis retinse ; of.n. optic nerve.
Before becoming enveloped in the optic nerve tins artery is continued through the vitreous humour (Fig. 128), and when it comes in close proximity to the lens it divides into a number of radiating branches, which pass round the edge of the lens, and form a vascular sheath which is prolonged so as to cover the anterior wall of the lens. In front of the lens they anastomose with vessels, coming from the iris, many of which are venous, and the whole of the blood from the arteria centralis is carried away by these veins. The vascular sheath surrounding the lens is the membrana capsulopupillaris. The posterior part of it is either formed simply by branches of the arteria centralis, or out of the mesoblast cells involuted with the lens. The anterior part of the vascular sheath is however enclosed in a very delicate membrane, the membrana pupillaris, continuous at the sides with the membrane of Descemet.
The membrana capsulo-pupillaris is simply a provisional embryonic structure, subserving the nutrition of the lens.
In many forms, in addition to the vessels of the vascular capsule round the lens, there arise from the arteria centralis retinae, just after its exit from the optic nerve, provisional vascular branches which extend themselves in the posterior part of the vitreous humour. Near the ciliary end of the vitreous humour they anastomose with the vessels of the membrana capsulo-pupillaris.
The choroid slit closes very early, and is not perforated by any structure homologous with the pecteri. The only part of the slit which can be said to remain open is that in which the optic nerve is involved ; in the Centre of the latter is situated the arteria centralis retinae as explained above. From this artery there grow out the vessels to supply the retina, which however are distinct from the provisional vessels of the vitreous humour just described, the blood being returned from them by veins accompanying the arteries. On the atrophy of the provisional vessels the whole of the blood of the arteria centralis passes into the retina.
Of the cornea, aqueous humour, eyelids and lacrymal duct no mention need here be made, the account given in Part I. being applicable equally to mammalian embryos.
The auditory organ
In Mammals, as we have seen to be the case in the chick (chap, vi.), the auditory vesicle is at first nearly spherical, and is imbedded in the mesoblast at the side of the hind-brain. It soon becomes triangular in section, with the apex of the triangle pointing inwards and downwards. This apex gradually elongates to form the rudiment of the cochlear canal and sacculus hemisphericus (Fig. 129, GO). At the same time the recessus labyrinthi (R.L) becomes distinctly marked, and the outer wall of the main body of the vesicle grows out into two protuberances, which form the rudiments of the vertical semicircular canals (V.E). In the lower forms (Fig. 132) the cochlear process hardly reaches a higher stage of development than that found at this stage in Mammalia.
The parts of the auditory labyrinth thus established soon increase in distinctness (Fig. 130); the cochlear canal (CC) becomes longer and curved ; its inner and concave surface being lined by a thick layer of columnar epiblast. The recessus labyrinthi also increases in length, and just below the point where the bulgings to form the vertical semicircular canals are situated, there is formed a fresh protuberance for the horizontal semicircular canal. At the same time the central parts of the walls of the flat bulgings of the vertical canals grow together, obliterating this part of the lumen, but leaving a canal round the periphery ; and, on the absorption of their central parts, each of the original simple bulgings of the wall of the vesicle becomes converted into a true semicircular canal, opening at its two extremities into the auditory vesicle. The vertical canals are first established and then the horizontal canal.
|Fig. 129. Transverse section of the head of a fetal sheep (16 mm in length) in the region of the hind-brain. (After Bottcher.)
HB. the hind-brain. The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuli R.L. and of the commencing vertical semicircular canal F.L., and of the ductus cochlearis CO., with the cavity of the primary otic vesicle are seen : on the left side, only the extreme end of the ductus cochlearis CC, and of the semicircular canal V.B. are shewn.
Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC ; on the left side the auditory nerve G' and its connection N with the hind-brain are also shewn.
Below the otic vesicle on either side lies the jugular vein.
|Fig. 130. Section of the Head of a Foetal Sheep 20 mm in Length (After Bottcher.)
R. V. Recessus labyrinthi ; V.B. vertical semicircular canal ; HE. horizontal semicircular canal ; C.C. cochlear canal ; G. cochlear ganglion.
Shortly after the formation of the rudiment of the horizontal semicircular canal a slight protuberance becomes apparent on the inner commencement of the cochlear canal. A constriction arises on each side of the protuberance, converting it into a prominent hemispherical projection, the sacculus hemisphericus (Fig. 131 8E).
The constrictions are so deep that the sacculus is only connected with the cochlear canal on the one hand, and with the general cavity of the auditory vesicle on the other, by, in each case, a narrow short canal. The former of these canals (Fig. 131 6) is known as the canalis reuniens.
At this stage we may call the remaining cavity of the original otic vesicle, into which all the above parts open, the utriculus.
Soon after the formation of the sacculus hemisphericus, the cochlear canal and the semicircular canals become invested with cartilage. The recessus labyrinthi remains however still enclosed in undifferentiated mesoblast.
Between the cartilage and the parts which it surrounds there remains a certain amount of indifferent connective tissue, which is more abundant around the. cochlear canal than around the semicircular canals.
|Fig. 131. Section through the internal ear of an embryonic sheep 28 mm. in length. (After Bottcher.)
D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior vertical semicircular canal ; U. utriculus ; H.B. horizontal semicircular canal ; b. canalis reunions ; a. constriction by means of which the sacculus hemisphericus S.lt. is formed ; /. narrowed opening between sacculus hemisphericus and utriculus ; C.C. cochlea ; C.C.L . lumen of cochlea ; K.K. cartilaginous capsule of cochlea ; K.B. basilar plate ; Ch. notochord.
As soon as they have acquired a distinct connectivetissue coat, the semicircular canals begin to bo dilated at one of their terminations to form the ampullae. At about the same time a constriction appears opposite the mouth of the recessus labyrinthi, which causes its opening to be divided into two branches one towards the utriculus and the other towards the sacculus hemisphericus ; and the relations of the parts become so altered that communication between the sacculus and utriculus can only take place through the mouth of the recessus labyrinthi (Fig. 132).
When the cochlear canal has come to consist of two and a half coils, the thickened epithelium which lines the lower surface of the canal forms a double ridge from which the organ of Corti is subsequently developed. Above the ridge there appears a delicate cuticular membrane, the membrane of Corti or membrana tectoria.
The epithelial walls of the utricle, the saccule, the recessus labyrinthi, the semicircular canals, and the cochlear canal constitute together the highly complicated product of the original auditory vesicle. The whole structure forms a closed cavity, the various parts of which are in free communication. In the adult the fluid present in this cavity is known as the endolyinph.
In the mesoblast lying between these parts and the cartilage, which at this period envelopes them, lymphatic spaces become established, which are partially developed in the Sauropsida, but become in Mammals very important structures.
They consist in Mammals partly of a space surrounding the utricle and saccule and called the vestibule, into which open spaces surrounding the semicircular canals, and partly of two very definite channels, which largely embrace between them the cochlear canal. The latter channels form the scala vestibuli on the upper side of the cochlear canal and the scala tympani on the lower. The scala vestibuli is in free communication with the lymphatic cavity surrounding the utricle and saccule, and opens at the apex of the cochlea into the scala tyrnpani. The latter ends blindly at the fenestra rotunda.
The fluid contained in the two scalse, and in the remaining lymphatic cavities of the auditory labyrinth, is known as perilymph.
The cavities just spoken of are formed by an absorption of parts of the embryonic mucous tissue between the perichondrium and the walls of the membranous labyrinth.
The scala vestibuli is formed before the scala tympani, and both scalse begin to be developed at the basal end of the cochlea : the cavity of each is continually being carried forwards towards the apex of the cochlear canal by a progressive absorption of the mesoblast. At first both scalse are somewhat narrow, but they soon increase in size and distinctness.
The cochlear canal, which is often known as the scala media of the cochlea, becomes compressed on the formation of the scalse so as to be triangular in section, with the base of the triangle outwards. This base is only separated from the surrounding cartilage by a narrow strip of firm mesoblast, which becomes the stria vascularis, etc. At the angle opposite the base the cochlear canal is joined to the cartilage by a narrow isthmus of firm material, which contains nerves and vessels. This isthmus subsequently forms the lamina spiralis, separating the scala vestibuli from the scala tympani.
The scala vestibuli lies on the upper border of the cochlear canal, and is separated from it by a very thin layer of mesoblast, bordered on the cochlear aspect by flat epiblast cells. This membrane is called the membrane of Reissner. The scala tympani is separated from the cochlear canal by a thicker sheet of mesoblast, called the basilar membrane, which supports the organ of Corti and the epithelium adjoining it. The upper extremity of the cochlear canal ends in a blind extremity called the cupola, to which the two scalse do not for some time extend. This condition is permanent in Birds, where the cupola is represented by a structure known as the lagena (Fig. 132, II. L). Subsequently the two scalse join at the extremity of the cochlear canal ; the point of the cupola still however remains in contact with the bone, which has now replaced the cartilage, but at a still later period the scala vestibuli, growing further round, separates the cupola from the adjoining osseous tissue.
Accessory auditory structures
The development of the Eustachian tube, tympanic cavity, tympanic membrane and external auditory meatus resembles that in Birds (p. 166). As in Birds two membranous fenestrse, the fenestra ovalis and rotunda, in the bony inner wall of the tympanic cavity are formed. The fenestra ovalis opens into the vestibule, and is in immediate contiguity with the walls of the utricle, while the fenestra rotunda adjoins the scala tympani. In place of the columella of Birds, three ossicles, the malleus, incus and stapes reach across the tympanic cavity from the tympanic membrane to the fenestra ovalis. These ossicles, which arise mainly from the mandibular and hyoid arches (vide p. 403), are at first imbedded in the connective tissue in the neighbourhood of the tympanic cavity, but on the full development of this cavity, become apparently placed within it, though really enveloped in the mucous membrane lining it.
|Fig. 132. Diagrams of the Membranous Labyrinth. (From Gegenbaur.)
I. Fish. II. Bird. III. Mammal
U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. canalis reuniens ; R. recessus labyrinthi ; UC. commencement of cochlea ; C. cochlear canal ; L. lagena ; K. cupola at apex of cochlear canal ; V. csecal sac of the vestibulum of the cochlear canal.
In Mammalia the general formation of the anterior and posterior nares is the same as in Birds; but an outgrowth from the inner side of the canal between the two openings arises at an early period ; and becoming separate from the posterior nares and provided with a special opening into the mouth, forms the organ of Jacobson. The general relations of this organ when fully formed are shewn in Fig. 133.
Fig. 133. Section through the Nasal Cavity and Jacobson's Organ. (From Gegenbaur.)
- n. septum nasi ; en. nasal cavity ; J. Jacobson's organ ; d. edgs of upper jaw.
The development of the cranial and spinal nerves in Mammals is as far as is known essentially the same as in the chick, for an account of which see p. 123 et seq.
Sympathetic nervous system. The development of the sympathetic system of both Aves and Mammalia has not been thoroughly worked out. There is however but little doubt that in Mammalia the main portion arises in continuity with the posterior spinal ganglia.
The later history of the sympathetic system is intimately bound up with that of the so-called supra-renal bodies, the medullary part of which is, as we shall see below, derived from the peripheral part of the sympathetic system.
The Elements of Embryology - Volume 2 (1883)
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