428 OUTLINES OF CHORDATE DEVELOPMENT 



amnion, and the entire amnion and chorion then have a meso- 

 dermal lining. In the rabbit the amnionic cavity remains 

 relatively small, while the exocoelom expands so as to fill the 

 cavity of the blastodermic vesicle, save for the space occupied 

 by the yolk-sac and allantois (Fig. 180). The tail- fold appears 

 extremely early in some forms, such as the mole (Talpa) and 



FIG. 178. Diagram of the embryonic membranes and appendages of the 

 Mammals in general. From Hertwig (Lehrbuch, etc.), after Turner. AC, 

 Amnionic cavity; al, allantois; ALC, allantoic cavity; am. amnion; E, embryonic 

 ectoderm; H, embryonic endoderm (the reference letter is placed in the gut 

 cavity); M, embryonic mesoderm; pc, trophoblast or trophoderm with villi; 

 sz, chorion; UV, yolk-sac. 



in Tarsius, so that the point of the final closure of the amnionic 

 folds is far forward over the head. The extent of the proam- 

 nion is also extremely variable; in some forms (e.g., Ruminants, 

 bats, many Primates) it is nearly or quite absent, while in 

 the opossum it forms nearly the entire amnion. 



Turning to the second type, where the amnionic cavity is 

 formed in situ, as a closed cavity from the very beginning, we 

 may describe the human amnion as an example. The earliest 

 stages in its formation have not been observed as yet, in man, 



