MUSEUM OF COMPARATIVE ZOOLOGY. 337 



disappear as soon as the nucleus has taken on the genuine spindle 

 shape. 



Selenka believes he can add to the results reached by Auerbach, Stras- 

 burger, and Biitschli. Soon after the segmentation nucleus has taken 

 an elliptical form, its substance is separated into a protoplasmic nuclear 

 spindle, and a nuclear fluid surrounding it. The latter is so difficult to 

 distinguish on account of the " clear area" which surrounds the nucleus, 

 that it is easily overlooked, and hence arises the erroneous idea that the 

 spindle represents the whole of the contents of the nucleus shrivelled. 

 The nuclear fluid increases rapidly in amount, assumes a spherical form, 

 and finally ruptures the nuclear membrane (already figured by Biitschli), 

 and thus becomes mingled with the surrounding yolk. 



During the fission of the nuclear plate a small lustrous corpuscle lies 

 at the tip of each end of the spindle. Whence it comes, Selenka cannot 

 say, nor can he add anything concerning the origin of the spindle fibres 

 or their thickenings ; but the segmentation nuclei of the second genera- 

 tion arise substantially from a fusion of the elements of the nuclear 

 plate, without the participation of any other formed structure. 



From this it may be inferred that the corpuscles at the tip of the 

 spindle do not enter (at least without first sufl'ering dissolution) into the 

 composition of the new nuclei. I fail to see, however, that the observa- 

 tions, so far as the figures allow one to judge (Taf. III. Figs. 18-20), are 

 competent to prove this exclusion. If the exclusion of these corpuscles 

 were established, and if they are identical with the corpuscles occupying 

 the centre of the polar areas in Limax, (an identity which I do not feel 

 to be fully justified in view of their prominently eccentric position in the 

 terminal suns,) then I should find in Selenka's statement a confirmation 

 of the opinion I have elsewhere ventured, viz. that the areal corpuscles 

 do not directly share in the formation of the new nuclei. 



The nuclear fibres (14-24 in number) are at first separated from each 

 other by equal intervals ; but when the fibre thickenings have advanced 

 to near the ends of the spindle, and have fused in pairs, these secondary 

 thickenings take the form of a circle [ring f\ and further unite, as has been 

 stated in the review of the preliminary paper. Instead of Vorkerne 

 used in the earlier communication, Selenka suggests the use of Nucleo- 

 plasts for the " Kernfaserelemente," since the former name (pronuclei) 

 is otherwise employed. When the " nucleoplasts " are six in number at 

 each pole, they have the form of conical rods with the tips directed out- 

 ward, their blunt ends united to the interzonal filaments. 



After the formation of the segmentation nuclei of the second genera- 



voL. VI. — NO. 12. 22 



