Book - An Introduction to the Study of Embryology 6

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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.

   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B
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Chapter VI. Organs Derived from the Hypoblast

In a previous section the archenteron was left as a simple sac or tube, opening to the exterior anteriorly by the stomodseum, and posteriorly by the proctodaeum.

From what was said concerning the effects of the presence of a large amount of food-yolk, it will be obvious that there will be a discrepancy in the relative time of the development of various hypoblastic structures ; for example, in telolecithal ova the ventral wall of a considerable portion of the alimentary canal must of necessity be completed very late.

The primitive function of the hypoblast is undoubtedly alimentation, but in the course of evolution it has acquired several other functions. The digestive organs will now be first considered, and subsequently other hypoblastic derivatives will be described.

Digestive Organs

The simple sac-like archenteron of the gastrula, as has already been described, is produced into pouches in a large number of animals.

When this occurs in Sponges the characteristic hypoblast cells (choano-flagellate cells) become restricted to the extremities (ciliated chambers) of the often complicated diverticula. All the exhalent canals are lined with flattened hypoblast cells.

The gastric diverticula of Coelenterates appear to be chiefly concerned with the circulation or distribution of the nutritive fluid, the actual process of digestion being probably confined to the stomach of the Hydromedusse, and the edges of the mesenteries in the Actinozoa (fig. 68).

In the Coelomata, or those animals provided with a true body cavity, these diverticula are cut off from the gastric cavity, and are henceforth spoken of as mesodermal structures.

The gastric diverticula of the Turbellarians, of certain Nemerteans, and of the Leeches, cannot be regarded as coelomic diverticula which have never severed their connection with the archenteron.

It has been shown (p. 29) that in most centrolecithal ova (e.g., Crustacea) some of the hypoblast cells engulf the food-yolk which lies within the segmentation-cavity (fig. 22). In other ova the yolk is originally located within the primitive hypoblast. In both cases it is digested by those cells.

The actual conversion of the primitive hypoblast into special digestive cells has not been fully investigated, but it must be readily effected, as digestion and assimilation are primary properties of protoplasm.

The hypoblastic portion (mesenteron) of the alimentary canal is always divisible into definite regions, and, with the exception of most of the Arthropoda, it forms by far the largest section of the tract.

The various regions of the alimentary canal of different animals which appear to be similar had received corresponding names before their development was known, consequently many apparent morphological anomalies must be expected.

Usually among the , Invertebrates the stomodseum is prolonged as the oesophagus ; the mesenteron includes the stomach and intestine and their associated glands, while the proctodeum is small. The Arthropoda, as a whole, are an exception to this rule, for in Insects the mesenteron is that portion of the alimentary canal lying between the crop or proventriculus, when that is present, and the point of origin of the Malpighian tubes. The mesenteron may be a simple tube, or divided into regions, of which the anterior may possess numerous small caeca (some Beetles) or eight large ones (Cockroach). In low forms, such as the Myriapoda and Peripatus, the mesenteron is long and simple.

In the lower Crustacea the mesenteron is relatively long. There are in Amphipods, in addition to the two or four digestive caeca, which are so commonly present throughout the Crustacea, two long narrow tubes which open into the extreme hinder end of the mesenteron. These tubes are undoubtedly excretory, but, as Spencer has shown, they are hypoblastic and not epiblastic, they cannot be regarded as homologous with the Malpighian tubules of the Tracheata (p. 111).

The mesenteron of the Decapod Crustacea is restricted to the usually minute chamber between the so-called pyloric chamber (fig. 140) and the commencement of the intestine (proctodaeum) ; it is separated from the former by valves. It is to this that the term stomach should be restricted. The digestive gland or so called “liver- opens by a wide aperture on each side into the mesenteron. The latter is the only portion of the alimentary canal of these animals which is not lined by cuticle.

In the Mollusca (figs. 1 8 and 84) only the buccal cavity is lined by epiblast, the stomach and intestine being archenteric derivatives. The stomodaeum gives rise to the buccal cavity and its organs (radula or odontophore, salivary glands), and to the oesophagus. The proctodaeum is very small. In the Cephalopoda the ink sac

Fig. 140. - Diagrammatic Sections of Embryos of the Cray-Fish (Astacus Fluviatilis). [From Huxley after Reichenbach.]

C. Longitudinal section of an ovum in which the rudiments of the abdomen, of the hind-gut, and of the fore-gut have appeared. D. Later stage of similar embryo. E. Longitudinal section of newly-hatched embryo.

a. anus ; e. eye ; ep.b. epiblast : f.g. fore-gut (stomodaeum) ; f.g\ its oesophageal, and f.g 2 . its gastric portion ; h. heart; li.g. hind-gut (proctodaeum) ; m. mouth; m.g. midgut, mesenteron, or archenteron ; v. yolk. The dotted portions in D and E represent the nervous system.

early grows out as a simple diverticulum from the ventral wall of the hinder end of the intestine.,

Invertebrate Digestive Gland or Liver

The large digestive gland associated with the mesenteron in the higher Invertebrates (Molluscs and Arthropods) is usually spoken of as a “liver.- As a matter of fact, it is now known to be a more universal digestive gland than its name would apply, and that it more closely corresponds in function with the Vertebrate pancreas, combining, as it does, the function of liver and pancreas, it has been appropriately termed the hepato-pancreas. It is a complex gland which typically develops from the wall of the mesenteron (fig. 140) in the usual manner, but, in some forms, the liver appears to be formed by a metamorphosis of the remnant of the yolk-cells which remain after the formation of the mesenteron (fig. 84, B, y).

Mesenteron of Chordata

The hypoblastic portion of the alimentary canal of the Chordata is divisible into the following regions: pharynx, oesophagus, stomach, and intestine (figs. 14 1,


The egg being yolkless in Amphioxus, the archenteron (fig. 57) is directly converted into the alimentary canal of the adult.

Fig. 141. - Isolated Alimentary Canal of Embryo Dog of Twenty-Five Days. Multiplied 5 diameters. [ From Kolliker after Bischoff .]

a. pharyngeal or branchial pouches ; 6. rudiment of laryngeal portion of the pharynx ; c. lungs ; d. stomach ;•/. liver; g. dorsal wall of the vitelline sac, with which the intestine still communicates by a large orifice (the umbilicus) ; h. rectum.

The inner white line indicates the hypoblast ; the surrounding dark border representing the splanchnic or visceral (mesoblastic) sheath of the alimentary tract. Compare with A, fig. 143.

The effect on the formation of the mesenteron by the presence at first of a small, and then of a gradually increasing amount of food-yolk, has already been described (p. 30). The constriction off of the digestive tract from the yolk-sac in telolecithal ova takes a comparatively long time, and not a few Fish are hatched with the yolk-sac still depending from their bodies. In fig. 141, which illustrates the isolated alimentary canal of an embryo Dog, viewed from the ventral surface, it will be seen that all the main organs have made their appearance while the umbilicus is still widely open (see also fig. 143). The neck of the yolk-sac gradually narrows to form the vitelline duct, and the first fold of the intestine (figs. 144, I; 143, c) occurs at the spot where the vitelline duct joins it. A diverticulum which occasionally occurs in Man in the lower part of the ileum is the persistent base of the vitelline duct ; and not unfrequently the proximal portion of the vitelline duct may persist in Birds as a short tube connected with the small intestine.


The pharynx probably extended along a considerable length of the body in the primitive Chordata, as is still the case in Amphioxus and Lampreys. The lateral walls were devoted to respiratory purposes, as will be described subsequently.

A deep ciliated groove, the endostyle, extends along the median ventral line of the pharynx (branchial sac) in Ascidians. The cilia work from before backwards and thus carry the mucus, which is secreted by the glandular cells of the endostyle, along with entangled food particles into the oesophagus.

The hypopharyngeal ridge of Amphioxus, with its glandular cells, has a similar function.

This region corresponds to the non-respiratory ventral portion of the pharynx of Balanoglossus.

Fig. 142. - Diagrammatic Longitudinal Section through the Head of a Larval Lamprey (Pefcromyzon. [From Claus after Balfour .]

Ab. optic vesicle; C. heart; cb. cerebellum ; c.h. cerebral hemisphere ; Chd. notochord; H. hypophysial (thyroid) involution ; inf. infundibulum ; ks. branchial pouches ; m.b. mid-brain ; md . medulla ; N. nervous system; O. stomodseum; 01. olfactory pit ; ot. auditory vesicle, represented as visible; pn. pineal gland (below which the optic thalamus is shown) ; v.cuo. ventral aorta; ve. velum. The oblique line between the velum and the first branchial pouch represents the left of a pair of ciliated grooves which converge on the median ventral line to meet the orifice of the thyroid.

A considerable groove is developed in the front portion of the floor of the pharynx in the larval Lamprey (fig. 142), and to a decreasing extent in higher forms.

We may therefore conclude that the ventral portion of the primitive pharynx was concerned in the transmission of food. The special mechanism by which this was effected afterwards degraded into the median element of the gland known as the thyroid body (see p. 183). It is possible that this change of function was correlated with the increase in size of the primitive Chordata and the consequent ability to eat larger prey. The latter, from their size, would not have the tendency to escape through the gill-slits, which minute organisms could easily do, and would further pass into the oesophagus without requiring the assistance of the ventral groove. The latter, owing to disuse, would naturally degenerate.

Throughout the Xchthyopsida the pharynx gradually becomes greatly shortened, as is also the case in Amphibia and Amniota.

(Esophagus. - The oesophagus calls for no special mention. It is a simple tube of variable length, which in some forms (Crocodilia and many Birds) has a ventral saccular dilatation or crop.


The oesophagus may pass imperceptibly or abruptly into the stomach. The stomach is usually a simple dilatation of the alimentary canal (figs. 141-144). Its exact form varies considerably, but it only becomes at all complicated in a few Mammals (e.g., Sloths, Cetacea, Buminants, some Marsupials and Bodents).

There is an instructive modification in the stomach of Buminants during growth. In the early foetus the relative size of the compartments and general form of the stomach are almost exactly those of the adult. After birth, owing to the milk-diet, the growth of the peptic stomach or abomasus is greatly in excess of that of the others; but as a herbivorous diet is acquired, the characteristic form of the adult stomach is re-acquired.

To secure increase of secreting surface without proportionate extent of superficies, crypts or pockets of digestive cells were developed forming simple glands. In time these became more complex, as was previously described for epiblastic glands (p. 106), the cells which actually secrete the digestive fluid being restricted to the blind extremities or alveoli of the gland.

Three types of such glands are found in Mammals ; the simple tubular crypts of Lieberkuhn in the small intestine. A gland with a non-glandular duct and a few simple tubules is illustrated by the peptic and pyloric glands of the stomach, and the glands of Brunner in the pylorus, while the liver and pancreas represent the most specialised form of gland.


The liver - in Amphioxus, alone of all Chordata, retains its primitive tubular form. It is the earliest hypoblastic gland to be developed, and it is relatively very large in foetal life. It appears to be entirely absent in Balanoglossus.

In some of the lower Vertebrata (Elasmobranchs and Amphibia) (fig. 99) the liver arises from a single ventral diverticulum from the intestine, which soon becomes bilobed. In Birds and Mammals (fig. 1 41) the liver appears to be bilobed from the first.

The incipient liver buds out into a local thickening of the splanchnic mesoblast, which thus becomes penetrated by a number of rod-like prolongations (hepatic cylinders) of the primitive diverticula. As a rule the hepatic cylinders appear to be solid, but in Elasmobranchs Balfour found that they are hollow, as they are also stated to be in Amphibia. A system of ducts appears in due course. The hepatic cylinders have the peculiarity, which is unique among glands, of uniting with one another at numerous points, thus forming a network within the meshes of which the enveloping mesoblast develops into blood-vessels.

The gall-bladder is simply an enlargement of, or a diverticulum from, the main duct of the liver. Its presence is very variable ; the number and position of the ducts of the liver opening into the intestine are also inconstant in various animals.

Pancreas. - The pancreas occurs very constantly among the Vertebrates. It is absent in the Cyclostomi and Perennibranchiate Amphibia, and rudimentary or absent in many Teleosts. The pancreas may be partially imbedded in the liver in Ganoids, and completely so in Siluroids. It first appears as a tubular outgrowth from the dorsal wall of the intestine, opposite to, but slightly behind, the diverticulum, which forms the rudiment of the liver. According to His, the pancreatic rudiment at first appears in front of the liver in the human embryo, and later shifts its position to behind that viscus (fig. 143, b-d). Hollow diverticula arise from the main duct, which continually subdivide. The surrounding mesoblast develops as usual into blood-vessels and connective tissue. In some cases two pancreatic diverticula have been observed.

Fig. 143. - Four Stages in the Development of the Human Alimentary Canal,


all. stalk of allantois; b.p. bursa pelvis; c. caecum; ep. epiglottis; g.e. genital eminence; k. kidney; l. liver; la. larynx ; l.d. duct of liver; Ig. lung; l.j. lower jaw; p. pancreas ; pr. proctodaeum ; R.p. Rathke -s pouch (hypophysial evagination), behind it in A and B is Seesfeel -s pouch ; st. stomach ; t. tongue ; tky. median rudiment of thymus- gland; tr. trachea; u. ureter; umb. umbilical vesicle; v.d. vitelline duct; W.d. Wolffian duct.

Intestine. - The intestine is the post-gastric portion of the mesenteron. It is always a straight tube in epabryos, and persists as such in many of the lower Chordata. In other forms it becomes variously looped, owing to its length exceeding that of the body cavity within which it lies.

The posterior portion of the intestine in the adult, but not in the embryo, is usually of markedly greater diameter than the anterior portion or small intestine ; it is known as the large intestine.

The secreting and absorbing surface of the alimentary canal is increased in the lowest Vertebrates by the development of a longitudinal fold projecting into the cavity of the intestine, which is known as the spiral valve.

The fold is slightly developed in the Cyclostomi, and reaches its highest state of development in some Elasmobranchs. It becomes less marked in the Ganoids, and traces of it may be found in the intestine of a few Teleosts. In no higher Vertebrate has it been definitely recognised. A similar fold is found in the intestine of some Ascidians ; such a fold may be compared with the typhlosole of certain Invertebrates (ex. Earthworm and Fresh-water Mussel).

Fig. 144. -Rough Diagrams Illustrating the Change in Relative Position undergone by the Digestive Tract in Mammals. [ From Landois and Stirling.]

6. colon ; o. vitelline duct ; r. rectum ; t. small intestine ; v. stomach.

Concomitantly, according to Wiedersheim, with the disappearance of the spiral valve in Fishes a number of hollow diverticula (pyloric caeca) make their appearance from the anterior region of the small intestine (duodenum). These are found in some Ganoids, in which group their development is not always inversely proportional to that of the spiral valve, and in most Teleosts, but in no other animals. Their function appears to be, in some forms, to increase the absorbing surface of the intestine, as a digestive function may be present or absent [Stirling, Macullum]. In a few Teleosts they occur side by side with the pancreas.

Those animals which possess a spiral valve have, in the main, an alimentary canal which pursues a straight course through the body cavity. In other forms (excepting Teleosts) the greater length of the intestine probably renders a spiral valve superfluous.

The relative length of the alimentary canal is largely dependent upon the nature of the food of the animal. This is well illustrated in the case of the Frog -s tadpole. When still subsisting upon its stored-up food-yolk, the alimentary tract retains its primitive straight course (figs. 98, 99). After the tadpole is hatched it commences to feed upon decaying vegetable matter, and the intestine grows to a great length, and is coiled up like a watch-spring. Later on the young Frog takes to an animal diet, and the intestine is relatively very much shorter, and is only slightly looped.

The valvulae conniventes of Man, and similar folds in other animals, also serve to increase the absorbing surface of the small intestine. The development of all these structures is too obvious to require description.

In Mammals the end of the large intestine, where it passes into the small intestine, is usually enlarged to form the csecum. In Man there is at first no csecum (fig. 143, A-c), then a simple conical projection appears (fig. d) ; later the csecum lengthens, but the terminal portion does not keep pace with the growth of the base, and consequently becomes much narrower in calibre. The basal portion eventually grows so large that it is commonly called the csecum, while the true csecum is designated as the vermiform appendix. Several of the stages in the development of the human csecum are permanently retained in the adult stage in certain Mammals. It is not known whether the so-called vermiform appendix of the Wombat is, as in the higher Primates, a remnant of an originally elongated apex of the true csecum.

In some Armadillos the csecum is distinctly bilobed, and in Cyclothurus didactylus there are two distinct cseca. In addition to a capacious true csecum, Hyrax possesses a pair of simple conical Cseca in the large intestine.

In most Birds there are two cseca of variable length at the commencement of the large intestine.

A csecum is usually stated to first appear in Eeptiles, where it never attains a large size ; but Huxley has described and Howes has figured a representative of it in the Frog.

A simple rectal gland is found in Elasmobranchs.

Endodermal Muscles. - Muscular processes arising from the endodermal cells have been demonstrated by Jickeli in Hydra; these run transversely round the body, as opposed to the longitudinal direction of the similar fibres of the ectodermal cells. Endodermal muscular fibres have been demonstrated in the Actiniae by the brothers Hertwig.

Respiratory Organs of Invertebrates

In but few Invertebrates does the alimentary tract function directly in respiration. The endoderm lining the general cavity of the body in Actinozoa is, however, probably largely concerned in respiration, especially in such forms as Edwardsia, Cerianthus, and Peachia, which live imbedded in the sand.

Respiration probably occurs all along the intestine in Proneomenia, and along the rectum in ETeomenia.

The anal respiration of many Crustacea is, as has already been stated (p. 109), really proctodseal.

The respiratory trees of most Holothuroidea are probably of hypoblastic origin. In other Echinoderms the ambulacral system is partially respiratory.

Chordata. - The anterior portion of the chordate mesenteron is mainly devoted to respiration ; this may appropriately be termed the branchial region, or, more shortly, the pharynx.

In most Chordata several pairs of wide lateral pouches arise from the sides of the pharynx and come into close contact with the external skin. There is apparently a slight invagination of the latter to meet the former ; an absorption of the applied membranes results in the formation of lateral slits (branchial or visceral clefts), by means of which the cavity of the pharynx is put into direct communication with the exterior.

Delicate processes of the hypoblastic epithelium covering the intermediate bars (branchial or visceral arches) constitute the gills or branchiae. These are richly supplied with blood by the branchial vessels (p. 226). True gills, however, are never developed in the Amniota at any period of life.

Almost invariably the anterior (hyomandibular) visceral cleft is the first to appear, the remainder appearing in order from before backwards.

The worm-like Balanoglossus has pharyngeal gill-slits which arise in the same manner as those of Vertebrates ; for a long time there is only one pair, hut subsequently they are repeated in pairs, increasing in number with the increase in the size of the body [Bateson]. The collar at the base of the proboscis grows backward as an opercular fold to a variable extent in different species of Balanoglossus, but it never extends beyond three gill-slits. The enclosed cavity is termed the atrial cavity by Bateson.

Van Beneden and Julin have shown that all Ascidians have but a single pair of visceral clefts, which arise as a pair of pharyngeal pouches met by corresponding epiblastic depressions. This condition is permanently retained by the interesting tailed form Appendicularia. In all other Ascidians the gill-clefts fuse together to form a single chamber (peribranchial cavity or atrium), which almost entirely surrounds the pharynx (branchial sac). It is probable that the atrial pore is the persistent opening of the fused gill-slits. The atrium may be formed more especially from the hypoblastic or the epiblastic portion of these clefts. The numerous and usually irregular orifices (stigmata) in the pharynx clearly do not correspond with the gill-slits of higher forms, but are merely secondary perforations. We may say, with these authors, “ the Tunicata are Chordata with a single pair of branchial clefts, while the Yertebrata are furnished with several, and the Cephalochorda (Amphioxus) with a great number.-

In Amphioxus also a single pair of gill-slits first makes its appearance. This is subsequently followed by a large number (70-100), which slant obliquely from before backward. In the young form the gill-slits open directly to the exterior, but they are eventually covered by a pair of dorsal folds of skin which grow downwards, leaving a space between themselves and the gillslits (the branchial chamber or atrium). The two flaps of skin meet below the body and fuse throughout their whole extent except at one spot, the branchial or atrial pore. It will be readily apparent that the branchial chamber of Amphioxus is by no means homologous with that of Ascidians.

The number of gill-clefts never exceeds eight pairs in the Yertebrata. There are seven in the Cyclostomi and in Hexanchus, eight in Notidanus (Heptanchus), but six in all other Elasmobranchii ; amongst the Teleostei a further reduction in the number of clefts occurs, owing to the suppression of the hyoid pair.

The first cleft succeeding the mouth is termed the hyomandibular or hyoid cleft (spiracle), as it lies between the mandibular and hyoid arches. The second is correspondingly the hyobranchial or first branchial, and is bounded by the hyoid and the first branchial arches. The remaining slits are the branchial clefts.

Dohrn finds that the pair of ciliated grooves which lie in front of the gill-pouches in the Lamprey (fig. 142) is developed in the same manner as the branchial pouches, but an external opening is never acquired. This supposed lost pair of visceral clefts is termed by Dohrn spiracular or thyroidean.

Primitively all the visceral clefts were undoubtedly respiratory in function, and in many Eiasmobranchs the mandibular border of the spiracle bears a rudimentary gill. In Chimaera, some Ganoids, and many Teleosts, the hyoid border of the second cleft possesses only a rudimentary gill (opercular pseudobranch), which undergoes all stages of degeneration amongst the Teleosts, all the anterior gill-filaments having atrophied. The posterior gills have a tendency to disappear in Teleosts, the greatest reduction occurring in

Amphipnous cuchia, in which one branchial arch alone bears branchial filaments.

The gill-clefts in the Cyclostomes and Elasmobranchs are left quite exposed on the surface of the neck, but in Chimsera, Ganoids, Teleosts, and Dipnoi they are protected by a fold of skin (operculum), supported by skeletal elements ; the branchiostegal membrane and its supporting skeleton are derivatives of the hyoid arch. In some forms the border of the operculum fuses with the skin of the body, merely leaving a small orifice on each side leading from the branchial chamber.

In Amphibia the hyoid pharyngeal pouch never communicates with the exterior, but persists as the Eustachian recess. In larval life four, or rarely three (some Urodela), branchial clefts appear. The first, second, and third branchial arches develop external gills which may be covered by epiblast. These usually atrophy, and internal, probably hypoblastic, gills are developed on each side of the three branchial clefts. The internal gills are always lost, but in some Urodeles the external gills are retained throughout life. Cope has recently stated that the Siren loses and then re-acquires its external gills. Other Urodeles, which normally lose their gills when adult, may, however, become oexually mature while still retaining their gills (Axolotl).

An opercular fold grows back from each hyoid arch in Anura, and fusing above and below with the skin of the body, envelops the gills within a branchial chamber. At first the branchial chambers open widely to the exterior by an orifice on each side ; these persist in Dactylethra, according to Huxley. In Bombinator and certain other forms the openings of the branchial chambers unite to form a single ventral orifice. In the majority of Anura (Rana, Bufo), the two branchial chambers communicate by a ventral canal, and the opening of the right chamber is closed up, leaving a single asymmetrical pore on the left side.

External gills are present in some Ichthyopsida, but they have already been alluded to (p. 109).

The external gill filaments of Elasmobranch embryos arise as simple elongations of the posterior lamellae of each arch, the anterior not elongating at all. Dohrn finds that yolk is present in these filaments and in their veins, and also in the posterior branchial vein and the efferent arteries, but never in the heart or in the branchial artery. It would thus appear that these elongated filaments serve also to absorb the yolk.

In none of the Amniota do the visceral clefts bear gills at any period of life. In all forms there are four pairs of clefts, the last two being very small in Mammals. The visceral arches between the clefts are well marked (fig. 145, k", k"'), each possessing a central artery ; hut in Mammals the last cleft is not bounded by a posterior arch. In Man, at least, none of the visceral clefts are actually perforated [His], and the fourth and fifth external visceral furrows are withdrawn into a fold or sinus of the neck (sinus prsecervicalis), (figs. 146, 147).

The visceral clefts close up and entirely disappear, with the exception of the first (hyoid or hyomandibular), which, as has already been described (151), persists as the Eustachian tube and tympanic cavity.

Fig. 145. - Head of Embryo Rabbit of Ten Days.

Magnified 12 diameters. [ From Kolliker.]

a. eye ; at. atrium or primitive auricle of the heart ; b. aortic bulb ; k\ k", k" -. first (mandibular), second (hyoid), third (xst branchial) visceral arch ; m. mouth ; 0. superior maxillary process, and u. inferior maxillary (mandibular) process of the right side ; s. mid-brain, which forms the interior extremity of the body ; v. anterior portion of head and fore-brain ; v. ventricle of the heart.

Intestinal Respiration. - Many Teleosts swallow atmospheric air, which passes along the alimentary canal and is ejected by the anus. There can be no doubt that this is a method of supplementary respiration. In these forms the hypoblast of the intestine is a respiratory tissue. Gage finds that the papillate mucous membrane of the pharynx of the American fresh-water Turtle, Aspidonectes spinifer, is distinctly respiratory in function, but this does not appear to hold good for other forms [Haswell].

Air-Bladder. - A tubular diverticulum grows out from the dorsal side of the oesophagus or stomach in most Ganoids (fig. 152, A, a.b) and Teleosts. In the Salmon and Carp [Yon Baer] it arises just in front of the liver, and slightly to the right side. It grows backwards, and in some cases forwards as well. Excepting in some Teleosts this structure persists as the air-bladder.

It is possible that the primitive diverticulum from the mesenteron, which afterwards developed into the air-bladder, was originally connected with secretion. A small sac of unknown function occurs on the dorsal wall of the gullet in some Elasmobranchs.

The gases contained in the alimentary canal, and also, perhaps, air swallowed for respiratory purposes, would naturally tend to collect in a dorsal diverticulum. A hydrostatic apparatus would thus be formed, the muscular walls of the sac (airbladder) enabling the quantity of the contained gases to be regulated.

In some Fish (Physoclysti) the duct (pneumatic duct) by means of which the airbladder communicates with the alimentary canal becomes closed ; and in others, as in the Pleuronectidse, the air-bladder may entirely disappear.

In the Physoclysti the amount of gas in the air-bladder is regulated by diffusion through a network of blood-vessels. Under some conditions the fish may respire with the air secreted in the air-bladder by its own blood-vessels ; but this is a purely accessory and temporary mode of respiration.

The air-bladder in some Ganoids and Teleosts, and notably in the Dipnoids, is cellular and very vascular, and atmospheric air is in some of them known to be sucked in through the mouth, so that the air-bladder functions like a true lung.

In Gurnards and other Teleosts the air-bladder is used in making grunting sounds. In many Teleosts the air-bladder functions as an accessory auditory organ, either by impinging directly on the vestibulum of the internal ear, or by being indirectly connected with it by means of a chain of ossicles. The auditory function is most highly developed in the Siluroids, in which group the air-bladder becomes strangely modified, and may come anteriorly into close contact with the body-wall immediately behind the shoulder-girdle. The body- wall may become extremely thin at this spot, so as to form a regular tympanum. It is interesting to note that this tympanic membrane, like the tympanum of the ear, is lined externally by epiblast and internally by hypoblast.

In no organ of Vertebrates is there so varied a change of function as there is in this enteric diverticulum of Fishes.

Lungs. - The lungs are developed from the ventral wall of the oesophagus immediately behind the pharynx as an elongated groove, which abruptly terminates posteriorly (fig. 143, A, Ig). This ventral groove becomes constricted off from the oesophagus, except at its anterior end (glottis), where it still retains its connection with the pharynx (fig. 143, la). The blind slightly swollen extremity of the newly formed tube is the rudiment of the lung, and the duct is the trachea.

The lung very early exhibits a bilobed character (figs. 14 1, c; 146, c, T). Some observers state that it is from the first distinctly paired.

In most Amniota the surrounding splanchnic mesoblast becomes greatly thickened, and the hypoblastic sac-like lungs burrow into the stroma, dividing and subdividing as they advance. Eventually an extremely ramified system of tubes is formed in Mammals, each ultimate branch of which being terminally distended into a sacculated ampulla (infundibulum).

The primitive sac-like character of the lungs (fig. 143, b) is retained in the Amphibia and most Reptilia, the walls being merely infolded to give increased respiratory surface.

In the Chameleons variable branched prolongations of the lungs project freely into the body cavity. Analogous diverticula appear in the embryos of Birds, and ultimately form the air-sacs. Prolongations from the latter pass into many of the bones in most Birds, the penetration of these delicate sacs into the bones being due to bone-absorption consequent on pressure.

The cartilaginous rings of the bronchi and trachea and the cartilages of the larynx are of mesoblastic origin.

The air-bladder of the Dipnoids is clearly homologous with the same organ of other Fishes, but in this remarkable group of animals the air-bladder is distinctly double ; its walls are greatly infolded (“spongy - or “ cellular -) and very vascular ; the blood supply is taken directly from the last aortic arch, and not from the caeliac artery, the blood being returned directly to the heart, and not to the liver, as in other Fishes ; lastly, the wide pneumatic duct opens on the ventral wall of the throat (the same also occurs in the Ganoid Polypterus). In all these points the air-bladder of the Dipnoi resembles the lungs of Amphibia. From these facts it is usually concluded that lungs are directly derived from the air-bladder of Fishes.

Minot, however, has suggested that the lungs have been evolved by the modification of a pair of gill-pouches, which do not break through in the neck, but grow down into the thorax (figs. 14 1, c, 146, c, l).

Albrecht considers it erroneous to homologise dorsal with ventral organs, and points out the difficulty of the migration of the dorsal air-bladder to a sub -oesophageal position. In the Gymnodont Teleosts, in addition to the dorsal air-bladder, there are ventral air-sacs proceeding from the oesophagus, by means of which these fishes can inflate themselves. These sacs are considered by him as homologous with lungs, and heterologous to the dorsal air-bladder. The air-bladder of Polypterus would therefore be the homologue of the lungs. Dorsal diverticula from the oesophagus opposite the larynx may normally (Pig) or abnormally (Man) be present.


Born finds that in the Pig the tongue is developed from the anterior portion of the ventral floor of the pharynx. The space between the ventral ends of the first and second visceral arches is at first depressed; but later a longitudinal ridge grows up, separated from the arches on each side by a groove. The anterior portion of this ridge grows forward and becomes the free part of the tongue. The tongue does not extend back beyond the second arch, but the posterior portion of the ridge projects between the third and fourth arches and develops the epiglottis. As Minot points out, the epithelium covering of the tongue is thus hypoblastic in origin.

If the above statement is correct, the taste-buds on the papillae are hypoblastic sense organs. The gustatory goblet-cells on the tongue of Amphibia possibly have a similar origin. The goblet-shaped organs in the mouth and pharynx of Fishes may have a similar function, but those of the mouth appear to be homologous with similar organs situated in the skin. Macullum has very recently recorded the occurrence of taste-buds in the oesophagus of the Sturgeon ; he also states that he has found them in Ampliioxus as far back as the opening of the hepatic caecum.

According to His, the tongue in Man has a double origin. From the anterior region of the ventral space (mesobranchial area), between the visceral arches of an early embryo, a small round projection (tuberculum impar) is formed ; behind this are a pair of folds (furcula), which eventually will form the epiglottis. The ventral portions of the second and third arches grow towards their fellows of the opposite side between the tuberculum and the furcula. The basal growths of the arches form the roots of the future tongue, and unite together behind the tuberculum impar;

Fig. 146. - Diagrams Illustrating the Visceral Arches and Development op the Tongue in the Human Embryo. [After His.]

Seen from above, the dorsal (posterior) wall of the pharynx being supposed to be cut away. In B. the branchial blood-vessels are indicated.

F. furcula ; l. lungs ; R. roots of the tongue ; s.pc. sinus praecervicalis ; T. tuberculum impar, body of the tongue in D ; 1-5. visceral arches.

the median pit between these structures is the rudiment of the median lobe of the thyroid body. The tongue is formed by the fusion of the two roots with the tuberculum.

Thyroid Body. - The generally received view of the significance of the thyroid body has already been mentioned (p. 172).

In the Lamprey (fig. 142) the thyroid body arises as a wide diverticulum from the floor of the anterior portion of the pharynx. The orifice becomes restricted to a pore and eventually disappears. During larval life it consists of a median ciliated portion communicating with a pair of complicated lateral glandular sacs.

In some higher forms the thyroid is stated to develop as a tubular diverticulum or solid down-growth from the anterior region of the pharynx, which later becomes bilobed. Subsequently it is quite detached from the pharynx, and is produced into a number of hollow or solid processes, between which connective tissue septa and blood-vessels enter.

Born reconciles various conflicting observations regarding the origin of the thyroid body in Mammals by finding that, according to his investigations, the organ has a double origin. An unpaired portion arises as an invagination from the floor of the pharynx opposite the front edge of the second visceral cleft. It separates from the pharyngeal epithelium, expands laterally, and migrates backwards. The other portion of the thyroid is derived from the paired remnants of the fourth visceral clefts. These are at first somewhat pear-shaped hollow sacs, but on becoming connected with the central portion they acquire a spongy interior. Pischelis confirms Born -s statements from his researches on the Pig, Rabbit, and Birds. His finds, in the human embryos, that the median thyroid rudiment arises as a hollow diverticulum between the third visceral arches, and that the lateral portions are evaginated from the posterior end of the pharynx near the glottis (fig. 147, l.thyr). The several parts become separated from their parent tissues and sink into the deeper portion of the neck. The duct of invagination of the median portion persists for some time as the ductus thyreoglossus (fig. 147, d.thyr). The foramen csecum, cornu medium, and the various bursse which may be present iq the adult are rudiments of this duct.

The last investigation on the thyroid body is that of De Meuron, who finds that the median element is always (Elasmobranch, Frog, Lizard, Fowl, Sheep, and Man) developed from a median pit in the pharynx at the level of the second visceral arch. He homologises the supra-pericardial bodies of Elasmobranchs [Van Bemmelen] and Amphibia [De Meuron] with the accessory or lateral thyroid bodies of the Amniota (the left alone occurs in Acanthias and Lacerta). The structure of both resembles that of the median element. These bodies arise as a pair of diverticula behind the sixth branchial cleft (seventh visceral), which is imperfectly developed in many Elasmobranchs. It may be concluded that these represent a degenerate pair of gills as in Heptanchus, in which there are seven branchial clefts, the supra-pericardial bodies are absent. In the higher Fishes and larval Amphibia the lateral rudiments of the thyroid develop directly from the pharynx behind the last (fifth branchial cleft. Owing to a further reduction of the clefts, which also disappear without leaving a trace and a consequent shortening of the pharynx, the lateral thyroids appear to develop from the fourth branchial cleft ; this is most marked in Mammals. The similarity in structure of the fully-developed lateral thyroids with the median element and their close connection in adult Mammals rather tend to support Dohrn -s hypothesis concerning the primitive condition of the median thyroid, i.e., that it represents a pair of degraded hyoid clefts.

Thymus Gland

Maurer finds that in the embryo Trout the thymus takes its origin from four thickenings of the epithelium of the visceral clefts on each side of the body, a rudiment being situated in the dorsal angle of each of the four clefts. A proliferation of the epithelium takes place, and the four rudiments on each side become fused together. Each lateral thymus gland sinks into the underlying mesoblast and takes on the character of a lymphatic gland.

Dohrn states that there is primitively one thymus rudiment for each branchial cleft in Elasmobranchs, but the fifth disappears in the Sharks. The separation of these rudiments from the epithelium is due to the shortening of the clefts and the bending of the visceral arches. The parts thus isolated gave rise to a new organ, the thymus, which was afterwards transmitted by heredity to higher Vertebrates.

In the Pig, according to Born, the thymus arises as a pair of ventral evaginations from near the inner openings of the third pair of visceral clefts, the outer portions of which atrophy. The end of the thyroid rests against the pericardium at the spot where the aorta leaves it. The central cavity disappears, and many branches grow out from the solid cord, mainly in the direction of the heart.

His finds in the human embryo that the primary (epithelial) rudiment of the thymus arises from the epithelium of the inner portion of the fourth, third, and partly also of the second visceral clefts. These parts become massed together and separated from the outer skin (fig. 147, thm). He asserts that it is developed from the epidermis and cannot be regarded as a hypoblastic structure, since in Man none of the visceral clefts become perforated.

It is evident that the function of the gland in the Amniota is secondarily acquired, and that it is a degraded epithelial organ, "which, from its relation to the gill-clefts in Fishes, may possibly have been some form of sense organ.

De Meuron has also studied the development of the thymus. In Ichthyopsida and Sauropsida it arises as solid thickenings of the epithelium of the dorsal side of the branchial clefts. In Fishes it arises from the first four branchial clefts, in the Lizard from the second, third, and fourth, and in Birds from the third and fourth visceral clefts. In the last three groups the thickening of the third cleft is the largest. In Anura the second visceral cleft alone develops a thymus. The history of the thymus is very different in Mammals ; dorsal rudiments are developed, as in Birds, from the third and fourth visceral clefts, but nearly the whole of the adult organ is derived from a ventral caecum from the third branchial cleft.

Fig. 147. - Development of the Thymus Gland and Thyroid Body in the Human Embryo. [After His.]

A. Transverse section through the hinder portion of the head. B. Transverse section through the larynx of an older embryo. C. Profile reconstruction of the thyroid and thymus glands, seen from below. D. The same seen from the side.

ao.a, ao.d. ascending and descending aorta ; c. carotid artery ; d.thyr. ductus thyreoglossus ; ep. epiglottis ;/.c. foramen caecum ;j.v. jugular vein ; l. larynx; l.thyr. lateral thyroid rudiment; m.thyr. median thyroid rudiment; ce. oesophagus; p.a. plicae aryepiglotticse ; ph. pharynx ; r.t. roots of the tongue ; t. tongue ; thm. rudiment of the thymus gland ; tr. trachea ; ix. glossopharyngeal ; x. vagus ; xi. hypoglossal ; xn. spinal accessory nerves ; 1-5. visceral arches.

Gustatory Organ of Amphioxus

The organ usually known as the olfactory organ of Amphioxus consists of an outer ciliated sac opening to the exterior and also into an inner sense-organ, which again communicates with the mouth. Hatschek finds that the whole organ is developed from the left of a pair of archenteric diverticula in front of the mouth, and that it is therefore of purely hypoblastic origin. It probably is an organ of taste. Hatschek and Dohrn are inclined to homologise it with the hypophysis.

Excretory Organs

The only excretory organs which appear to be of hypoblastic origin are the paired urinary tubes which occur in Amphipods. They arise from the extreme hind-end of the mesenteron, there being a distinct break of continuity when the latter ceases and the hind-gut (rectum) begins [Spencer]. Their development is unknown.

Skeletal Structures. - Notochord. - The primitive axial supporting rod or skeleton (notochord), or chorda dorsalis, which is peculiar to the Chordata, and from which they derive their name, is of hypoblastic origin.

Hemichordata. - Bateson has shown that in the larval Balanoglossus (B. kowalevskii) the median dorsal wall of the pharynx is constricted off and grows forward as a short hollow diverticulum of hypoblast, which afterwards becomes solid except posteriorly, where its lumen opens throughout life into the pharynx. The cells scon become vacuolated as in the notochord of higher forms.

Urochordata. - In Ascidians the notochord is developed solely in the tail, it being derived from the dorsal wall of the caudal arclienteron.

Cephalochordata. - In Amphioxus the notochord is, as it were, pinched off from the median dorsal wall of the archenteron (fig. 56 nch). Ultimately its folded appearance and its connection with the archenteron are lost. It is constricted off from before backwards.

Vertebrata. - In the lower Vertebrates the notochord is distinctly derived from the dorsal wall of the mesenteron (archenteron). Hertwig -s researches on the development of the Newt (Triton) show that the dorsal hypoblast (usually referred to in this book as invaginated hypoblast and the chorda entoblast of Hertwig and others) not only is distinctly columnar, as opposed to the rounded ordinary hypoblast cells, but it also lines a distinct groove (fig. 148). The two sides of the notochordal groove, as it may be termed, come together and form a solid rod of cells, the arrangement of which gives no indication of their origin. The notochordal groove is scarcely apparent in the Frog.

Mitsukuri and Ishikawa have demonstrated that the notochord in the Snapping Turtle (Trionvx japonicus) (fig. 149) is developed in a manner perfectly comparable with that of the Newt. Indications of a similar origin of the notochord are found in Lizards, and notably in Mammals.

In Birds the axial hypoblast very early becomes converted into the rudiment of the notochord, and this may occur almost before the permanent hypoblast can be recognised as such; hence the supposition of some authors that the notochord was derived from the mesoblast.

Fig. 148. - Transverse Section of the Dorsal Portion oe an Embryo Newt (Triton). [ 4 /f«‘ O. Hertivig.]

a. mesenteron ; ax.hy. axial hypoblast in process of forming the notochord; b.c. coelom (body-cavity) ; ep. epiblast ; hy. digestive hypoblast ; n.p. neural plate ; so.m. somatic mesoblast; sp.m. splanchnic mesoblast.

The rudiment of the notochord consists of a solid rod of cells lying between the neural tube and the mesenteron. Posteriorly it is connected with the fusion of the layers which occurs at the

(Fig. 149. - Formation of Notochord in Trionyx. [ After Mitsukuri and Ishikawa.]

A. Transverse section through the head region before the closure of the neural groove B. D. Portions of successive sections of the same embryo. The shading of the epiblast is purely diagrammatic.

am. amnion ; ax.hy. axial hypoblast ; ep.a. epiblastic, and hy.a. hypoblastic layer of amnion; hy. hypoblast; to. mesoblast; n.c. neural canal: nch. notochord.

dorsal lip of the blastopore (fig. 62), or, when there is no distinct blastoporic passage, as in the Fowl, it passes into the primitive streak. At a later stage the notochord terminates anteriorly behind the infundibulum, its extremity being often recurved. Posteriorly the notochord terminates at the end of the tail.

A definite sheath (elastica limitans interna) is soon formed as a secretion from the peripheral cells of the notochord. The cells of the notochord become vacuolated, so that the notochord has a spongy appearance ; a few nuclei surrounded by a little protoplasm remain attached to some of the meshes of the network (figs. 150, 152, 173, 175, ch).

The notochord and its sheath are replaced in most Vertebrates, leaving only a small rudiment, as will be mentioned in the description of the development of the vertebral column (pp. 196-199).

Sub-Notochordal Rod. - A solid rod of cells is developed from the dorsal wall of the alimentary canal in Ichthyopsida after the formation of the notochord (figs. 150, 173, 175, x).

This sub-notochordal rod, as it is termed, has about the same extension as the notochord. Its function or homology is unknown, but it appears to persist as the sub-vertebral ligament in the Sturgeon.

Significance of the Notochord

Few embryological problems are more obscure than the probable phylogenetic significance of the notochord. The embryological evidence points to its hvpoblastic origin. We are justified in assuming the primitive, or at all events the archaic, nature of its development in the Amphioxus (fig. 56) and the Newt (fig. 148). The variations which are met with in other Vertebrates can be reduced to the type of the Newt, as is proved by the Chelonia (fig. 149).

The development of the urochord in the Ascidians is manifestly a degraded process.

The restricted notochord of Balanoglossus develops in an essentially similar manner to that of Amphioxus, but the central lumen is retained for a much longer period. It is interesting to note that in some Amniota a transient canal occurs at the posterior end of the notochord.

Upon an examination of the figures given by authors illustrating the development of the notochord in Balanoglossus, Amphioxus, the Newt, Chelonia, Lizards, and Mammals, the conclusion seems to be almost inevitable that we must regard the notochord as a secondary structure. It may be that the ancestor of the Chordata possessed a longitudinal groove along the neural aspect of its alimentary tract, which may have had some special secretory (? mucous) function. The extremely early acquisition of distinctive histological characters may be recalled in this connection.

The closure of the notochordal groove in ontology at the time of the constriction off of the archenteric diverticula from the mesenteron is suggestive of phylogenetic synchrony.

It is not difficult to imagine that a rod of cells, even though containing at first a small lumen, might form a mechanical support to the body which would prove of considerable value, and, being internal, it would grow with the growth and requirements of the animal.

Urinary Bladder

The urinary bladder is properly speaking a hypoblastic organ, but it is more convenient to deal with it at the same time as the uro-genital ducts (P- 2 59)

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   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2024, April 17) Embryology Book - An Introduction to the Study of Embryology 6. Retrieved from

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