CHAPTER VIII. 



ACCESSORY EMBRYONIC ORGANS; UMBILICAL VESI- 

 CLE, AMNION, AND ALLANTOIS. 



THUS far the process of development relates to the principal parts 

 of the body of the embryo. In some species this includes aU the 

 important structures in the impregnated egg; the embryo arriving 

 very soon at a stage of growth in which it is capable of an independent 

 existence. But in many fish and reptiles, and in all birds and mam- 

 malia, additional structures are produced, which aid in the protection 

 or nutrition of the embryo during the middle and later periods of its 

 development. In these instances certain portions of the blastoderm, 

 like those forming the area vasculosa in the fowl's egg, remain outside 

 the body, and assume the function of accessory organs. The most 

 important of these are the umbilical vesicle, the arnnion, and the 

 allantois. 



Umbilical Vesicle. 



In the frog's embryo (page 620), the abdominal plates, closing to- 

 gether in front, join each other upon the median line ; thus shutting in 

 the vitellus, and enclosing it in the future intestinal canal. 



In other instances the abdominal plates do not immediately embrace 

 the whole of the vitelline mass, but approach each other at some inter- 

 mediate point ; constricting the vitellus and dividing it by this means 

 into two portions, one of which is included 

 within the body of the embryo, while the 

 other remains outside (Fig. 192). As de- 

 velopment proceeds, and the embryo in- 

 creases in size, the constriction becomes 

 more strongly marked, forming a nearly com- 

 plete separation between the internal and 

 external portions of the vitellus. The in- 

 ternal portion remains as part of the in- 

 testinal canal; while the external portion, EGG OF FISH, showing formation 



of umbilical vesicle. 



with its blastodermic covering, forms a sac- 

 like appendage to the abdomen, attached at the umbilicus, and known 

 as the umbilical vesicle. 



The umbilical vesicle is accordingly lined by a portion of the internal 

 blastodermic layer, continuous with the epithelium of the intestine ; 

 and covered by a portion of the external blastodermic layer, continuous 

 with the integument of the abdomen. 



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