94 COMPARATIVE PHYSIOLOGY. 



V 



than the other. The allantois may fuse with adjacent mem- 

 branes and form at one part a condensed and hypertrophied 

 chorion (placenta), with corresponding atrophy elsewhere. 

 The arrangement of the placenta varies in different groups of 

 animals so constantly as to furnish a basis for classification. 

 Whatever the variations in the structure of the placenta, it is 

 always highly vascular ; its parts consist' of villi fitting into 

 crypts in the maternal uterine membrane — ^both the villi and 

 the crypts being provided with capillaries supported by a con- 

 nective-tissue matrix covered externally by epithelium. The 

 placenta in its different forms would appear to have been 

 evolved from the dififuse type. 



The peculiarities of the embryonic membranes in birds are 

 owing to the presence of a large food-yelk, egg-shell, and egg- 

 membranes ; but throughout, vertebrates follow in a common 

 line of d.evelopment, the differences which separate them into 

 smaller and smaller groups appearing 'later and later. The 

 same may be said of the animal kingdom as a whole. This 

 seems to point clearly to a common origin with gradual diver- 

 gence of type. 



