MUSEUM OF COMPARATIVE ZOOLOGY. 405 



dus, Rhabditis dolichura, Diplogaster, and Succinea, there appear only two 

 such nuclear structures ; in Cephalobus, at the poles, but not always at 

 the saaie time ; in Rhabditis, one at the vaginal pole, the other at the point 

 of the surface where the vesicle disappeared, whether the equator or nearer 

 the vaginal pole. The migration and union of these two nuclei, except 

 in Cephalobus, is less regular than Auerbach represents it to be. The 

 amoeboid motion of, the yolk at this time is sufficient to explain the mi- 

 gration, but not the coalescence, of the nuclei. In the cases of Rhabditis, 

 Cucullanus, and Lymnseus there are, however, more thaii two nuclear 

 structures, — from three to eight, or even more, according to the animal. 

 In Cucullanus these arise close under the surface at points remote from 

 each other ; in all cases they are at first quite small, and successively 

 unite till a single nucleus results. In the author's opinion the formation 

 of the nucleus of the first segmentation sphere by the union of several 

 nuclear structures, since it is of wide-spread or general occurrence, refutes 

 Auerbach's idea that the whole process results from fecundation taking- 

 place at a definite pole of the Qgg, and the same is shown even more con- 

 clusively by the fact that the nuclei of later generations also arise from 

 the union of several nuclear structures. 



GoETTE ('75, pp. 20 - 22) has described the regressive metamorphosis 

 of the germinative vesicle in the case of Bombinator igneus. After 

 approaching the upper (dark) pole of the yolk, it is found to have suf- 

 fered diminution of volume, so that it no longer fills completely the 

 cavity in the yolk which it once occupied. The remaining space of this 

 cavity is filled with a clear fluid. At a later stage there appears at the 

 centre of the dark pole, therefore directly over the vesicle, an irregular 

 yellow spot, which is found, on making sections, to be due to an inter- 

 ruption or obliteration of the pigment layer, which thus allow^s the un- 

 colored yolk to come to the surface. It is found at the same time 

 that the cavity about the shrunken vesicle has disappeared. Goette 

 concludes that the former appearance is caused by the escape of the clear 

 fluid of this cavity, whereby the integrity of the pigment layer is inter- 

 rupted. There are still some traces of the germinative dots and of the 

 membrane of the vesicle. A little later there is to be found in place of 

 the vesicle only an exceedingly fine-granular substance without definite 

 limits. All this takes place before the eggs are laid, therefore indepen- 

 dently of fecundation. 



In freshly laid eggs certain changes in the region of the yellow spot 

 are to be observed. But only a few of the fresh eggs exhibit all the 

 phenomena. The middle of the spot is often depressed, and sometimes 



