ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 43 



14. The nuclear division of this primitive cell does not occur in 

 any constant direction, but in a direction conditioned by the adjacent 

 vacant space. 



Mechanism of Fertilization.* — Pfeffer's observations on the 

 attraction of spermatozoa in cryptogams, suggested to Herr J. 

 Dewitz the study of sperm movements with the view of detennining 

 how far their entrance into the ovum was effected by chance. His 

 observations were made on spermatozoa of Periplaneta (Blatta) 

 orientalis, and they led him to the following results: — (1) The 

 spermatozoa are attracted to surfaces, e. g. were found moving on the 

 cover-glass and on the slide, but not in the space left between them, 

 or round the walls of a hollow glass ball, but not in tlie centre ; 

 (2) the spermatozoa move in a circle, and, for the observer, in 

 the direction of the figures on a watch-dial. Therefore, in actual 

 fertilization, Herr Dewitz maintains that the sperms are drawn to the 

 surface of the ovum, move round it in slightly varying circles as 

 above noted, and thus must reach a micropyle. Experiments on 

 pieces of the egg-shells, the eggs themselves being too opaque, con- 

 firmed this opinion. In regard to mammals, he maintains a chemical 

 attraction. 



Blastodermic Vesicle of Mammals.f — Prof. A. C. Haddon sug- 

 gests the view that in the blastodermic vesicle of mammals, at the 

 close of segmentation, the inner mass, since it gives rise to the embryo 

 proper, is perfectly comparable with the germinal disc of a fowl 

 during the later stages of segmentation, which has sunk into the 

 blastodermic vesicle owing to the absence of yolk. The outer layer 

 corresponds to those epiblast cells which are gradually inclosing the 

 yolk, the so-called blastopore of van Beneden indicating in an ex- 

 aggerated manner the distinction between the embryonic and non- 

 embryonic germinal layers. Epiblast cells grow over this " blasto- 

 pore " and form the covering cells ; eventually the invagination of 

 the germinal area is rectified, and there is a diploblastic ovum, the 

 covering cells forming the spurious third layer which misled van 

 Beneden. 



The segmentation of the ovum is next discussed, and the conclu- 

 sion is arrived at, that the first immigration of blastospheres into the 

 interior of the ovum (van Beneden's stage 3) indicates the gastrula 

 stage. It would further appear that this immigration was asymmetrical, 

 much as there is an asymmetrical invagination of the hypoblast in 

 telolecithal ova. The extension of cells of the blastodermic vesicle 

 over the embryonic area is probably to be accounted for in most 

 cases by the sinking of the latter into the cavity of tlie former. 

 These covering-cells are really a portion of the blastodermic vesicle, 

 that is of the yolk-sac, and they form the first adhesion between the 

 ovum and the parent. This is compared with the imperfect attach- 

 ment of the embryos of marsupials to the uterine walls, which is 



* Arch. f. d. Gesammt. Physiol. (Pfliiger), xxxvii. (18S5) pp. 219-23. 

 t Proc. R. Dublin Soc, iv. (1885) pp. 536-47 (7 figs.). 



