252 



COMPARATIVE ZOOLOGY OF CHORDATES 



underlying maternal connective tissue is invaded by the 

 developing villi of the trophoblast, so that the latter comes 

 into contact with the walls of the maternal capillaries. Sub- 

 stances have only to pass through the wall of the maternal 

 capillaries, the trophoblast, and the wall of the embryonic 

 capillaries in order to diffuse through. This type of placenta 

 occurs in carnivora (cat and dog), and is restricted to a zone of 

 the trophoblast, whence its name zonary. 



(iv) The epithelium of the uterus is removed, but the 

 underlying connective tissue is not invaded as in the carnivores ; 

 instead the trophoblast is very much thickened and then 



°J 



eh njc +• f- mb. e f 



Fig. 119. — Section through a part of the allantoic placenta of the cat ; 

 embryonic tissue to the left, maternal to the right. 



Letters as Fig. 118. 



hollowed out here and there to form lacunae. The remaining 

 projections from the trophoblast are called pseudovilli to 

 distinguish them from the true villi which are definite out- 

 growths. The maternal blood-vessels are " tapped " by the 

 very thorough erosion of the uterine wall, and the blood 

 flows out of them and into the lacunae in the trophoblast. 

 The pseudovilli are therefore bathed in the blood of the 

 mother, and the substances have only to pass through the 

 trophoblast and the wall of the embryonic capillaries to enter 

 into the embryonic circulation. This is the highest type of 

 placenta, and it is found in the rabbit, mouse, bat, shrew, 

 hedgehog, mole, Tarsius, monkey, and man. It is interesting 



