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LXXIV.— NOTE ON THE BLASTODEEMIC VESICLE OF MAM- 

 MALS. By ALFEED C. HADDON, M.A., M.E.I.A., 

 Professor of Zoology, Eoyal College of Science, Dublin. 



[Read, June 15, 1885.] 



Having recently liad occasion to refer to the development of 

 mammals, it occurred to me that the significance of the so-called 

 " blastopore of van Beneden " might be capable of elucidation. 

 I therefore beg to offer the following note on the subject which 

 may tend towards that, object. 



The following is a brief account of the segmentation of the 

 rabbit's ovum, as described by Ed, van Beneden (3, 4, 6) : — 



Within an hour or two after fertilization the ovum commences 

 to segment ; the whole process of segmentation occupies from 

 seventy to seventy -five hours, by which time the ova have just 

 entered into the cavity of the uterus. 



The ovum divides into two nearly equal spheres ; the larger is 

 clearer and somewhat differently acted upon by reagents than the 

 other. Yan Beneden believes that this indicates a fundamental 

 difference between the two cells ; the larger cell representing, ac- 

 cording to him, the " ectoderm," and the smaller granular cell the 

 " endoderm." In Quain's ElemenU of Anatomy, vol. ii. (9th edi- 

 tion, 1882), p. 744, they are .spoken of as "upper" and "lower" 

 spheres respectively ; but in the description of the figures (figs. 

 630-1) "ectomere" and " entomere," Heape adopts the preferable 

 plan of simply terming them and their derivatives " outer" and 

 "inner" spheres. 



Further segmentation into four and into eight spheres occurs 

 in the usual manner. As is usually the case in holoblastic ova, 

 the early-formed spheres are more or less spherical, and directly 

 the third stage has been completed, the eight spheres lie in two 

 tiers of four each in the ordinary manner ; but very shortly one of 

 the inner spheres passes into the interior of the mass, and the 

 whole ovum regains its spherical contour. 



The fourth stage is marked by the segmentation of the outer 

 cells only, the blastula now consisting of eight outer cells and four 



