CHAPTEE IX. 



DEVELOPMENT OF ACCESSORY ORGANS IN THE 

 IMPREGNATED EGG. UMBILICAL YESICLE, AM- 

 NION, AND ALLANTOIS. 



THUS far, the process of development has been followed as it relates 

 to the primary formation of the principal parts of the body of the 

 emb^o. In some species of animals this includes all the important 

 structures which show themselves in the impregnated egg; the embryo 

 arriving ver} r soon at a stage of growth in which it is liberated by the 

 rupture of the vitelline membrane and is already capable of carrying on 

 an independent existence. But in many fish and reptiles, and in all 

 birds and mammalia, there are additional structures which aid in the 

 nutrition of the young animal during the middle and later periods of its 

 development. In these instances, the whole of the blastoderm is not 

 immediately converted into the tissues of the embryo. Certain por- 

 tions, both of its external and internal layers, remain outside the limits 

 of the body, and perform, in this situation, the function of accessory 

 organs. These organs are the umbilical vesicle, the amnion, and the 

 allantois. 



Umbilical Vesicle. 



In the frog's embryo (page 725), the abdominal plates, closing to- 

 gether in front, join each other upon the median line, and shut in directly 

 the whole of the vitellus, which is thus inclosed in the intestinal sac 

 formed by the internal blastodermic layer. 



In other instances, the abdominal plates do not immediately embrace 

 the whole of the vitelline mass, but tend to close together at some inter- 

 mediate point ; so that the vitellus is con- 

 p' stricted, and divided into two portions, one 



internal, and one external. (Fig. 253.) As 

 development proceeds, the body of the embryo 

 increases in size out of proportion to the 

 vitelline sac, and the constriction just men- 

 tioned becomes at the same time more strongly 

 marked; so that the separation between the 

 internal and external portions of the vitelline 

 EGG OF FISH, showing for- gac is near ] y complete. The internal blasto- 



mation of umbilical vesicle. J 



dermic layer is by this means divided into 



two portions, one of which forms the intestinal canal, while the other, 

 remaining outside, forms a sac-like appendage to the abdomen, known 

 by 'the name of the umbilical vesicle. 

 (738) 



