90 . COMPARATIVE PHYSIOLOGY. 



The placentation. of certain of our domestic animals may be 

 thus expressed in tabular form (Fleming) : 



'General. |sow.' 



Simple placenta. 



I Cat, 



Local and circular. | ^i*"''- 



(Cow. 

 Multiple placenta. ■< Sheep. 



Comparing the formation, complete development, and atro- 

 phy (in some cases) of the various foetal appendages in mam- 

 mals, one can not but perceive a common plan of structure, 

 with variations in the preponderance of one part over another 

 here and there throughout. In birds these structures are sim- 

 pler, chiefly because less blended and because of the presence 

 of much food-yelk, albumen, egg-shell, etc., on the one hand, 

 and the absence of a uterine wall, with which in the mammal 

 the membranes are brought into close relationship, on the other; 

 but, as will be shown later, whatever the variations, they are 

 adaptations to meet common needs and subserve common ends. 



MICROSCOPIC STRUCTURE OF THE PliACENTA. 



This varies somewhat for different forms, though, in that 

 there is a supporting matrix, minute (capillary) blood-vessels, 

 and epithelial coverings in the foetal and maternal surfaces, the 

 several forms agree. 



The pig possesses the simplest form of placenta yet known. 

 The villi fit into depressions or crypts in the maternal uterine 

 mucous membrane. The villi, consisting of a core of connective 

 tissue, in which capillaries abound, are covered with a flat epi- 

 thelium; the maternal crypts correspond, being composed of 

 a similar matrix, lined with epithelium and permeated by 

 capillary vessels, which constitute a plexus or mesh-work. It 

 thus results that two layers of epithelium intervene between 

 the maternal and foetal capillaries. 



The arrangement is substantially the same in the diffuse and 

 the cotyledonary placenta. 



In the deciduate placenta, naturally, there is greater compli- 

 cation. 



In certain forms, as in the fox and cat, the maternal tissue 

 shows a system of trabeculse assuming a meshed form, in 

 which run dilated capillaries. These, which are covered with 



