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Zoology. — "On the placentation of Sciurus vulgaris." By Dr. 

 F. Muller. (Communicated bj Prof. A. A. W. Hubrecht). 



(Communicated in the meeting of September 29, 1906). 



I. T h e very earliest stages. The ovule of Sciurus under- 

 goes its first developmental stages in the oviduct. Meanwhile the 

 bicornuate uterus has prepared itself for the reception of the ovule: 

 underneath the single layer of epithelium the mucosa, which meso- 

 metrially remains very thin, has become very strongly thickened, 

 so that an excentrical T-shaped slit is left open, the transverse part 

 of which lies closest to the mesometrium. A special arrangement 

 for the attachment of the ovules can nowhere be detected ; a sub- 

 epithelial zone is found to be richer in nuclei, however, than the 

 loose connective tissue, separating this layer from the muscularis. 



II. P r e-p 1 a c e n t a r y stages (From the arrival of the ovule 

 in the uterus until the first formation of the allantoid placenta). 



The ovules fix themselves in varying numbers, to the right generally 

 more than to the left, at about equal distances on the anti-mesometral 

 (i.e. anti-placentary) uterine wall ; they are fixed with their vegetative 

 poles. A pellucid zone is absent, on the other hand the ovule becomes 

 surrounded by a mass, formed from glandular secretions of cellular 

 origin from the uterine wall. 



The ovules grow pretty quickly, for the greater part by dilatation 

 of the umbilical vesicle, which in these stages still forms tlie principal 

 part of the ovule. It is remarkable that the area vasculosa remains 

 so small, so that only entoderm and trophoblast form the wall of the 

 germinal vesicle over the greater part of the umbilical vesicle. 



The uterine wall shows intense activity during this stage. Many 

 processes take place here in rapid succession and simultaneously. 

 They all start from the spot where the ovule has settled, and from 

 this point extend in all directions, successively reaching the spaces 

 of the uterine honi, left open between the fixations of the o\ules, 

 as also the mesometrally situated parts ; all these processes begin 

 sub-epithelially, gradually penetrating deeper and deeper. These 

 successive processes thus gradually give rise to dish-shaped layers 

 of varying structure, surrounding the ovule at the anti-mesometral 

 side and the character of which is most sharply pronounced in the 

 points that are at the greatest distance from the mesometrium. ]iy 

 the extension of the anti-mesometral part of the long end of the 



