ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 621 



Formation of Placenta of Rabbit.* — We have again an account of 

 the development of the placenta of the Eabbit, M. J. Masius now being 

 the author. Previously to the formation of the blastocyst the mucous 

 membi"ane of the uterus becomes very thick, and forms on its siu-face 

 large papillae ; these, which are separated from one another by narrow 

 crypts, form a large projection on which the epiblast is fixed and the 

 placenta developed. Neither the glands nor the epithelium of the uterus 

 take any part in the formation of the placenta. The vessels of the 

 mucous membrane become surrounded by a sheath of cells which are all 

 derived from the fixed cells of the dermis ; these sheaths are developed 

 by the mitosic division of the cells which constitute them, acquire a 

 large size, and form the dominant mass of the dermis in the advanced 

 stages of the formation of the placenta. The endothelium of the vessels 

 of the mucous membrane degenerates and disappears ; when it disappears 

 the perivascular sheath directly bounds the cavity of the vessel and the 

 blood may filter in small quantities through the perivascular cells. 



In the early stages the mucous membrane contains a large number 

 of elements which the author regards as leucocytes ; these become con- 

 verted into corpuscles formed of a moniliform chromatic cord or of 

 chromatic granulations. These cells are scattered in the dermis of the 

 mucous membrane and in certain vessels, but the author does not yet 

 know what their function is. 



Just before the fixation of the blastocyst to the mucous membrane 

 two layers may be distinguished in the embryonic epiblast ; the deeper 

 one has cylindrical cells, the outer has nuclei arranged in nuclear 

 nests and there is no division into cellular areas. It is by means of this 

 superficial layer that union is made with the uterine mucous membrane ; 

 the layer soon becomes enormously developed and forms a multi- 

 nucleated protoplasmic mass into which the deeper epiblastic layer 

 sends primordial papillae, at first non-vascular and foimed of epiblast 

 and somatopleure. On the other hand, the capillaries of the mother 

 become engaged in this multinucleated protoplasm of fcetal origin, 

 where they lose their endothelium, and become continuous with a 

 system of numerous lacunae without proper walls. 



The allantois, by fusing with the serosa of Yon Baer, vascularizes 

 the primordial papillae by forming in them a connective axis rich in 

 blood-vesst Is. But, as tlais fusion is eff"ected, the deep layer of the 

 epiblast becomes interrupted around this connective axis of allantoidean 

 villosities. As a result of this, the maternal blood of the placenta 

 circulating in large lacunar spaces is, in many places, separated from the 

 connective vascular villosity by nothing more than a more or less thick 

 layer of multinucleated epiblastic protoplasm. 



The author regards the placenta of the Eabbit as a new formation of 

 foetal origin formed by allantoic villosities branching in a tissue wliich 

 arises solely from the ej)iblast of the embryo. This new formation 

 becomes fused with the dermis of the mucous membrane, the vessels 

 of which form quite a system of lacunae without proper walls, which 

 traverse a multinucleated protoj)lasmic mass not broken up into cell-areas, 

 and owing its origin to a very great increase in thickness of the suporiicial 

 layer of the epiblast. In the placenta the maternal blood circulates in 

 an epiblastic mass of embryonic origin. 



* Arch, cle Biologic, ix. (1889) pp. 83-121 (4 pis.). 



