Chap, xiii.] AMNION; ALLANTOIS. 509 



which the most common are the vitelline and the 

 zona radiata ; in some cases a third, more internally 

 placed membrane, is also developed. In addition to 

 these, there appear in the Sauropsida and Mammalia 

 two very important foetal membranes ; the first of 

 these forms an enveloping and protecting sac for the 

 embryo, and is called the ainiiion; it owes its 

 origin to two folds, which appear one at either end of 

 the embryo, rise up, curve over, and finally unite in 

 the middle line above. As each fold is double it 

 follows that, when the ends of each unite, two 

 envelopes are formed, and a cavity or space, which 

 later on becomes filled with the liquor amnii, is 

 developed between them. The inner of these envelopes 

 forms the true amiiion, and the outer the false 

 amnion or subzoiial membrane (Fig. 211) . 



The other covering is the allaiitois, and it has a 

 very different history. Early in the development of 

 the intestine there is given off from in front of the 

 region of the future anus a saccular outgrowth, which 

 gradually extends beyond the body of the embryo, 

 and over the greater part of it ; during the period of 

 foetal development this allantois remains connected by 

 a stalk with the body of the embryo, and over its 

 surface there extends a rich supply of blood-vessels, 

 the allantoic arteries and veins ; so that it has a 

 nutrient or a respiratory function, or both. 



In the Sauropsida all the material for the develop- 

 ment of the embryo is contained within the egg itself, 

 which, at a very early period, becomes sharply divisible 

 into a blastoderm, the seat of the future essential 

 stages in development, and a yolR sac, or store of 

 nutriment, the material from which is acquired by the 

 embryo through the intermediation of the rich plexus 

 of vessels which are rapidly developed within and 

 around it. As the yolk sac diminishes, the allantois, 

 for a time, increases in size, its vessels attain to 



