318 BULLETIN OF THE 



The metamorphosis o.f the primary segmentation nucleus into a spin- 

 dle was not so satisfactorily traced in the case of Cucullanus elegans. 

 Possibly a stage in this change is represented, says the author, in Fig. 

 20, where, in place of a nucleus, there is only an indistinctly defined 

 clear spot in the centre of the yolk, within which spot a number of dark 

 granular rods are irregularly disposed. This and all subsequent spindles 

 differ from the " Richtungsspindel " in that the rods of the nuclear plate 

 in the former lie within a definitely limited body, and cannot therefore 

 be simply a differentiation in the yolk (p. 224), while in the latter the 

 nuclear plate is formed of only a circle of dark granules (p. 226). 



When the new nuclei have made their appearance in the place of the 

 lateral plates, the nuclear fibres are no longer to be seen. Each nucleus 

 arises from a few (two to four) separate nuclei, which subsequently 

 unite. A distinct nucleolus is found only at a much later stage. The 

 radial structure of the yolk is to be seen during segmentation, as in 

 other eggs, but on account of the extremely fine-granular nature of the 

 yolk it is relatively difificult of observation. 



In his studies on the gasteropods Limnseus and Succinea, one looks 

 for a close agreement with the phenomena which take place in Limax. 

 The division of the primary segmentation nucleus begins in the still 

 spherical yolk by the appearance of two small radial systems at diamet- 

 rically opposite points of the nucleus, which determine the axis of the 

 subsequent division. 



In Biitschli's Fig. 10, Taf. IV., which is here cited, one sees that the 

 stellate figures arise at points on the nucleus which are not, strictly 

 speaking, diametrically opposite, but rather somewhat nearer the centre of 

 the e^g than is the centre of the nucleus ; just as we have seen in Limax 

 that the asters are somewhat deeper than the centre of the two pronuclear 

 structures taken as a whole. The centre of each star is occupied by a 

 homogeneous clear area. Since acetic acid was used by Biitschli, it is 

 not strange that a central, more refringent structure was overlooked. 

 Other reagents would doubtless have disclosed the fact that this area is 

 not entirely homogeneous. In a subsequent stage (Fig. 11) the nucleus 

 has assumed a streaked appearance. At first the dark interior corpus- 

 cles of the nucleus are visible between the streaks, but they soon disap- 

 pear, and the nucleus becomes a longitudinally striate spindle, stretching 

 between the two suns. Biitschli has figured (Fig. 13) an egg in which 

 the constriction of the yolk is conspicuously advanced, and in the mid- 

 dle of the spindle are seen fibre-thickenings, which he considers to he 

 the nuclear plate. Although it is hardly safe to infer from a comparison 



