164 



at any rate I find, on a renewed exatnination of iny prepara- 

 tions, that the two cavities in question are still in free commu- 

 nication at a date later than the stages he has there figured. 



Nussbaum's paper is a preliminary one, and is unaccompanied 

 by any figures. He also attributes a 'deciduai' origin to the 

 megalokaryocytes, and appears to think that the trophoblastic 

 lacunae are lined by maternal endothelium. 



Klebs has described one stage in the placentation of the rat. 

 His paper hardly contains au accurate statement; as examples of 

 his errors it may be mentioned that he identifies the proximal 

 wall of the yolk-sac with the allantois, and that he seems in- 

 clined to derive the distal wall from cells of the discus proligerus. 



Other Rodentia. 



a.) The Rabbit. 



The placentation of the Rabbit has received a very consider- 

 able amount of attention from embryologists, partly, no doubt, 

 because material is easy to obtain; but partly also because this 

 Rodent has long served as a classical type of the development 

 of a Mammal, with results that have not been altogether fortunate. 



The only complete account of the placenta is that of Duval 

 (6), which we shall accordingly review first. Other papers, deal- 

 ing principally with the early periods of development, have been 

 published by Masquelin and Swaen (30), Masius (29), Strahl (41), 

 Kossmann (25), Maximow (31, 32), and Marchand (27). 



In the situation of the placenta, and the arrangement of the 

 foetal membranes the Rabbit agrees closely with other Rodents. 

 There is a large, iuvaginated, yolk-sac with an area vasculosa 

 on the proximal wall, serving as an organ of nutrition up till 

 the moment of birth. By the disappearance of the distal wall, 

 and overlying trophoblast, the proximal wall is brought into 

 intimate relatiou with the anti-mesometric surface of the uterus; 

 the uterine epitheliura here degenerates. There is, however, no 

 capsularis, as there is in the mouse, and the allantois contains a 

 large endodermal cavity. 







