366 SUMMARY OF CURRENT RESEARCHES RELATING TO 



observed be of general occurrence, it will follow tbat "just as the 

 products of the division of sperm-forming cells are equivalent, so also 

 the nuclei arising from tbe division of the directive spindle will contain 

 equivalent material." Herr Platner does not discuss the relation of the 

 homology which he emphasizes to the various theories of polar globules, 

 but such application will doubtless be forthcoming. 



Segmentation in Double Organisms.* — Prof. G. Born has been 

 investigating the conditions of segmentation in ova which give rise to 

 double monsters. Starting from the conclusion of Eoux and Pttiiger, 

 that the first segmentation of the frog's ovum divides the material into 

 symmetrical halves which correspond to the future right and left sides, 

 Born first supposed that a duplicity would be evident from the beginning 

 in cases where double monsters arose. He sought for material, but the 

 monstrosities were too rare in frogs, and the eggs of Salmonidse were too 

 opaque. Pike ova, however, suited his purpose. With different females 

 the percentage of double monsters varies greatly from 3 to • 2 or • 5 

 per cent. His first year's experiments have not led him very far, but it 

 appears certain that " those ova which give rise to double monsters form 

 a first segmentation cleft single and regular, as in those from which 

 ordinary embryos develope." Born believes, however, that when the 

 double eggs, as we may call them, divide first into two, and then into 

 six portions, there must have been to start with two primary segmenta- 

 tion nuclei and two germinal vesicles. In such cases a double fer- 

 tilization must also occur. In ordinary segmentation he maintains that 

 the nuclei which divide to form right and left, or anterior and posterior 

 centres, are not congruent, but at most symmetrical. In double 

 monsters, he supposes that the first division is entirely congruent, that 

 the two nuclei are absolutely equivalent, that the differentiation into 

 right and left or fore and hind portions is only effected in the second 

 division. The appearances are the same as in the normal segmentation, 

 but their import is different. As to ova which at once divide into three 

 and four, they always perish, and probably illusti-ate the result of 

 polyspermy. 



;8, Histology.f 



Structure of the Cell and Phenomena of its Division. J — Herr G. 

 Platner commences his essay wdth an account of his observations on 

 cell- division and spermatogenesis in the hermaphrodite gland of Limax 

 agrestis. The secondary nucleus, first observed by la Vallette St. 

 George, is not the peculiar body it has hitherto been supposed to be, but 

 must be ranked with the " spheres attractives " described by van 

 Beneden in the cleavage-cells of Ascaris megalucepJiala, with the " archo- 

 plasm " of Boveri, and the " periplasts " of Vejdovsky. The author 

 agrees with van Beneden in thinking that similar elements will be found 

 in all cells. 



Spermatogenesis and cell-division in Paludina vivipara and Helix 

 pomatia is next considered. All the constituents of the sperm-producing 

 cells are oriented towards the centrosoma, which is contained in the 

 secondary nucleus. In cell-division the achromatic spindle, and then 

 the centrosomata with the primary rays of the polar radiate figures, are 



" JB. Schles. Ges., Ixv. (1888) pp. 79-90. 



t This section is limited to papers relating to Cells and Fibres. 



X Aich. f. Mikr. Amd., xxxiii. (1889) pp. 125-52 (2 pis.). 



