STUDIES IN MAMMALIAN EMBRYOLOGy. 487 



blast, and enter into communication with the maternal spaces 

 that have been laid bare after the disappearance of the maternal 

 epithelium. 



{d) The trophoblastic protuberances that have penetrated into 

 the crypts are hollowed out. AUantoidean villi enter into these 

 cavities. 



(e) The allantois sends new villi against the cytoblast, which, 

 however, do not continue to growcentrifugally. The cytoblast 

 itself grows centripetally^the peripheral portions being gradually 

 transformed into plasmodiblast, the central portions at the same 

 time ensheathing the newly formed villi. The latter are thus, 

 while gaining in length, enclosed by an identical trophoblastic 

 matrix (in which maternal blood circulates), as are their earliest 

 predecessors. 



(/) As the placentary region increases in breadth, space is 

 gained for the free development of these secondary villi. The 

 maternal proliferation at the same time flattens out to a super- 

 ficial covering of the growing placenta, and is finally reduced 

 to isolated nuclear remnants. 



(g) In the final stage of the placenta the allantoidean villi are 

 no longer recognisable as such, and the intervening trophoblastis 

 stretched to the utmost ; consequently there is only the thinnest 

 layer of plasmodiblast tissue to separate the maternal blood 

 fluid from the embryonic. 



Trophoblastic lacunae containing maternal blood can be very 

 easily distinguished from those spaces in which embryonic blood 

 circulates by the fact of the much smaller size of the maternal 

 blood-corpuscles. 



In the ripe placenta the most intricate intermixture of these 

 two sets of spaces is thus detected with great facility, whereas 

 the details of the genetic history of the placenta that are here 

 brought forward can by this important detail be easily traced 

 and tested. 



The recapitulation here given shows that the placenta is 

 essentially an embryonic neo-formation, which is per- 

 meated by maternal blood that circulates in spaces devoid 

 of endothelium. This embryonic neo-formation is preceded by a 



