THE YOLK-SAC. 



63 



Som 



Coe 



Another type is illustrated by the epithelial bodies, which are small masses of com- 

 pact cells resulting from a local epithelial growth, and penetrated by blood-vessels 

 (sinusoids) this type includes the parathyroid, nodulus thymicus, and post- 

 branchial body. 



The first gill-pouch becomes the Eustachian tube, the blind distal end being 

 expanded into the tympanum. 



The second pouch is partly obliterated, but its ventral part is converted into 

 the tonsil. 



The third pouch forms an epithelial body, the nodulus thymicus (Fig. 194, Nod) 

 and its ventral caecal prolongation is converted into the thymus. Its epithelium 

 is said also to produce a parathyroid. 



The fourth pouches give rise to a pair of parathyroids and to the post-branchial 

 bodies, which develop from the ventral prolongations of the pouches. 



The thyroid gland begins as a median evagination of the entoderm on the 

 ventral side of the pharynx. It starts very early Spl 



(human embryo of 3 mm.). The blind end of the 

 evagination becomes first bilobed, then branching 

 the branches are the anlages of the adult fol- 

 licles. The duct of the gland is soon obliterated, 

 but its point of origin is often permanently marked 

 by the foramen caecum at the back of the tongue. 



The Yolk-sac. 



General Morphology. The yolk-sac is the con- 

 tainer of the nutritive yolk destined to be assim- 

 ilated by the embryo. The principal factor in 

 its morphological constitution is the entoderm, 

 which, after the differentiation of the definitive 

 germ-layers, contains nearly all of the yolk mate- 

 rial. In the primitive vertebrates, as exemplified 

 by the marsipobranchs, ganoids, dipnoi, and am- 

 phibia, we find this yolk material lodged in the C<B > Coeiom. in, Intestinal cavity. Som, 



walls Of the primitive digestive tract. It is Situated Somatopleure. Spl, Splanchnopleure. 



chiefly on the ventral side of this tract and extends from the point where the heart 

 is formed toward the tail of the embryo to the point where the allantois is formed. 

 In other words, it is situated in a region ^corresponding to the territory of the 

 future abdominal cavity. In the primitive types just referred to, the yolk-bearing 

 entoderm becomes divided into distinct cells which form a large mass. The con- 

 dition may be understood from figure 44, which represents a transverse section of 

 the early stage of an axolotl embryo. The cavity of the entodermal canal (digest- 

 ive tract) is small. It is bounded on its dorsal side by a single layer of cells 

 distinctly epithelial in their development, and on the ventral side by a great mass 



FIG. 29. DIAGRAMMATIC SECTION OF 

 THE YOLK OF A HEN'S EGG AT AN 

 EARLY STAGE TO SHOW THE RELATION 

 OF THE PRIMITIVE ENTODERMAL 

 CAVITY, Ach. 



