82 



COMPARATIVE PHYSIOLOGY. 



a characteristic body, allantoin, related to uric acid, urea, 

 etc. 



Certain 'bodies, being probably inspissated allantoic fluid, 

 have been termed "hippomanes." They may either float free 

 in the fluid or be attached to the allantois by a slender pedicle. 



The relation of the parts described above will become clearer 

 after a study of the accompanying cuts and those of preceding 

 pages, in -which the aUantois is figured. 



Pig. 84.— Exterior of chorial sac; mare. (Clianveaa.) A, tody; B. C. comna. 



The Placenta. — This structure, which varies greatly in com- 

 plexity, may be regarded as the result of the union of structures 

 existing for a longer or shorter period, free and largely inde- 

 pendent of each other. With evolution there is differentiation 

 and complication, so that the placenta usually marks the site 

 where structures have met and fused, differentiating a new or- 

 gan; while corresponding atrophy, obliteration, and fusion take 

 place in other regions. 



All placentas are highly vascular, all are villous, all dis- 

 charge similar functions in providing the embryo with nourish- 

 ment and eliminating the waste of its cell-life (metabolism). 

 In structural details they are so different that classifications of 

 mammals have been founded upon their resemblances and dif- 

 ferences. They will now be briefly described. 



In marsupials the yelk-sac is both large and vascular; the 

 allantois small but vascular; the former is said (Owen) to be 

 attached to the subzonal membrane, the latter not ; but no villi, 

 and consequently no true chorion, is developed. All mammals 



