774 SUMMARY OF CURRENT RESEARCHES RELATING TO 



gained admission, it was only to disappear. The results of previous 

 investigators as to the penetration of the sperm are critically reviewed. 



The sperm head draws ever nearer to the germinal vesicle, and is 

 still followed by the tail. Within the germinal vesicle certain 

 changes have occurred ; the nuclear elements no longer exhibit uni- 

 form colouring, and have become rounded off fnto nucleolus-likc 

 " karyosomata." These scorn at first as if they were colourless, but 

 careful observation shows that the chromatin substance, formerly 

 diffusely distributed, is concentrated in the form of small granules at 

 the periphery. These chromatin elements fuse together afterwards, 

 and thus become more distinct. The karyosomata are, at first, 

 frequently united by unstained bridges, but these subsequently dis- 

 appear. The contour of the germinal vesicle has meanwhile changed 

 to an irregular one. Its position is still near the periphery of the 

 ovum at the animal pole. 



The head of the sperm and the surrounding clear space come to 

 rest within an insinking on the side of the germinal vesicle. In some 

 cases, all trace of a nuclear star, which always indicates activity of 

 some sort, was absent. The chromatin substance of the head soon 

 splits longitudinally. The connection between head and tail is at 

 length severed. Each of the two halves of the head is seen as a 

 round or oval hody within the germinal vesicle. Two of the karyo- 

 somata of the segmentation nucleus are markedly different from all 

 the others, and may be plausibly regarded as the two sperm elements. 



In association with the segmentation nucleus, there is at first 

 only one aster, which lies towards the interior of the egg, so that the 

 nucleus is between it and the periphery of the yolk. The connection 

 of aster and nucleus, the appearauce of a second aster and further 

 changes are described in detail, and the results up to this point are 

 again collated with those of previous investigators. 



The further phenomena of division are similar to those which 

 Tlatner has described in the sperm-forming cells of Helix. Tho 

 chromatin grains of the equatorial plate split longitudinally. The 

 opposed couples of granules into which the plate is resolved retreat 

 to the opposite poles. There the constituent elements fuse again 

 into spherical form. At the animal pole first, but soon over the 

 whole yolk, a furrow appears, becoming ever deeper, and dividing 

 yolk and spindle into two halves. The rest of the memoir contains 

 an account of the further steps in segmentation, and some further 

 notice of the spermatogenesis. 



Tooth-plates of some Stylommatophora.* — Dr. W. Dybowski 

 has investigated, mainly for diagnostic purposes, the tooth-plates of 

 some Gasteropoda belonging to the Stylommatophora division. The 

 median plate of the Pulmonata is throughout so essentially similar, 

 that it is of but little use in diagnosis. Two characteristic forms of 

 internal lateral plates are distinguishable ; (a) those in which the prin- 

 cipal tooth is simple (Succinea, Helix, Avion) ; (b) those in which the 

 principal is provided with a distinct median lateral tooth (Limax, 



* Bull. >Soc. Imp. Nat. Moscou, lxi. (ISbG) pp. 5U-63 (3 pis.). 



