740 ACCESSORY ORGANS IN IMPREGNATED EGG. 



the egg ; especially as the time occupied in the formation of the pri- 

 mary organs is very short, and the greater part of the process of de- 

 velopment remains to be accomplished after the young animal leaves 

 the egg. 



But in birds and quadrupeds, the time required for the development 

 of the embryo within the egg is longer. The young animal acquires a 

 more perfect organization during the time that it remains inclosed by 

 its membranes ; and the processes of absorption and exhalation neces- 

 sary for its growth, being increased in activity to a corresponding 

 degree, require a special organ for their accomplishment. This organ, 

 destined to bring the blood of the foetus into relation with the atmo- 

 sphere and external sources of nutrition, is the allantois. 



In the frog, the internal blastodermic layer, forming the intestinal 

 mucous membrane, is everywhere inclosed by the external layer, form- 

 ing the integument. But in the higher animals a portion of this internal 

 layer, which is the seat of the greatest vascularity, and which is des- 

 tined to produce the allantois, is brought into contact with the external 

 membrane of the egg for purposes of exhalation and absorption ; and 

 this can only be accomplished by opening a passage for it through the 

 external blastodermic layer. This is done in the following manner by 

 the formation of the amnion. 



Soon after the body of the embryo has begun to be formed, by the 

 thickening and involution of the external blastodermic layer, a second- 

 ary fold of this layer rises up on all sides about 

 Fig. 255. the edges of the newly-formed embryo ; so that 



its body appears as if sunk in a kind of depres- 

 sion, and surrounded with a membranous ridge, 

 as in Fig. 255. The embryo (c) is here seen in 

 profile, with the external membranous folds, above 

 mentioned, rising up in advance of the head, and 

 behind the posterior extremity. The same thing 



takeS P laCC On the tw sides f the foetllS > ^ the 



formation of lateral folds simultaneously with the 

 *PP<^ance of those in front and behind. As 

 these folds are destined to form the amnion, they 



* Called tlle " amni tic folds '" 



The amniotic folds continue to grow, extend- 

 ing forward, backward, and laterally, until they approach each other at 

 a point over the back of the embryo (Fig. 256) : Their opposite edges 

 afterward come in contact with each other at this point, and adhere 

 together, so as to shut in a space (Fig. 256, b) between their inner sur- 

 face and the body of the embryo. This space, which contains a thin 

 layer of clear fluid, is the amniotic cavity. 



There now appears a prolongation or diverticulum (Fig. 256, c), 

 growing from the posterior portion of the intestinal canal, and follow- 

 ing the course of the amniotic fold which has preceded it ; occupying, 

 as it gradually enlarges and protrudes, the space left vacant by the 



