484 CHORD ATA. 



It must be remembered that these types of placenta, 

 depending for distinction upon the degree of quantitative 

 or qualitative production of the villi, are gradational. 



The amnion remains as a thin membrane enveloping the 

 embryo and containing the liquor amnit, a colourless indif- 

 ferent fluid. Its walls are said to contract rhythmically and 

 rock the embryo. At birth the amnion is ruptured and its 

 remains are thrown off with the placenta. 



The yolk-sac, as before stated, never contains yolk and, 

 after the prokalymma has ceased to be functional, it either 

 shrivels and folds up at the lower pole or its outer wall, the 

 prokalymma, is shed, and the inner wall remains as a 

 vascular membrane. 



In this general account there are a series of striking 

 differences from the lower types (the Sauropsidd)^ and yet at 

 the same time there is a great degree of similitude. Here 

 are the same four foetal organs, the serosa, the amnion, the 

 allantois and the yolk-sac (or umbilical vesicle), but their 

 origin and function are different. The actual development 

 of the embryo and its organs is very similar, although it 

 commences much later. 



The main differences are as follows : — 



MAMMAL. SAUROPSIDA. 



1. The egg is minute (^^7^ in. i. The ^gg is large (about i inch 

 diam. in rabbit) with httle or no yolk diam. in chick), has a mass of yolk 

 and segmentation is total and equal. and segmentation is partial. 



2. The development of the em- 2. The development of embryo 

 bryo is very slow, but that of the commences first and that of mem- 

 membranes is rapid, hence the ser- branes is slower, hence the mem- 

 osa, amnion and yolk-sac are formed branes have their mesoblastic 

 at first without their mesoblastic sheaths from their inception, 

 sheaths. 



3. The serosa early becomes an 3. The serosa remains simple but 

 attaching organ, possibly also nutri- is covered by a porous shell, the 

 tive, and the yolk-sac never contains yolk-sac contains plentiful yolk, and 

 yolk, but it becomes an organ for the allantois is a respiratory organ, 

 interchange of blood, hence a nutri- its cavity forming an excretory re- 

 tive, excretory and respiratory organ, servoir. 



to be replaced in the Eutheria by 

 the allantois similarly modified. 



How are we to explain these important differences ? 

 The lowest mammals or Monotremata have large eggs, with \ 



