47° 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



...V 



. sp.m. 

 end. 



y 



¥ 



blood-vessels (w). The final im- 

 portant step in the development 

 of the placenta is that the ecto- 

 placental protoplasm becomes 

 thinned out into a barely recog- 

 nizable film and in places dis- 

 appears entirely and the same 

 happens to the connective tissue 

 surrounding the embryonic blood 

 vessels. When this stage has 

 been reached the embryonic 

 blood-vessels are in immediate 

 contact with the maternal blood 

 in the large sinuses, the two 

 streams of blood — ^embryonic and 

 maternal — ^being separated only 

 by the very thin wall of the em- 

 bryonic vessels, with it may be 

 sparse remnants of ectoplacenta 

 and connective tissue, through 

 which oxygen and food-material 

 readily pass from maternal blood 

 into embryonic, and carbon 

 dioxide and other excretory pro- 

 ducts in the opposite direction. 



The placenta when com- 

 pletely formed is a large dis- 

 coidal cushion composed partly 

 of uterine lining and partly of 

 embryonic tissue. It is connected 



Fig. 198. 



Diagrams illustrating the development of 

 the placenta of the Rabbit. (Based upon 

 Duval.) A, ninth day ; B, tenth day ; C, 

 twelfth day. Each diagram represents a 

 small portion of ectoplacenta, with the 

 adjacent embryonic and maternal tissue. 

 The ectoplacenta is shown in dark tone, 

 embryonic connective tissue in medium tone, 

 and maternal (uterine) connective tissue in 

 pale tone, coel, Coelome ; e, ectoplacenta ; 

 eni, embryonic endoderra ; s.m, somatic 

 mesoderm ; sp.m^ splanchnic mesoderm ; 

 V, uterine blood-vessels ; 11, embryonic blood- 

 vessels. 



