MUSEUM OF COMPARATIVE ZOOLOGY. 577 



cited (PI. I. Fig. 13) is the only one which shows such a condition, and even in 

 this one of the small nuclei is so covered by the other that the proof of their 

 independence is not conveyed by the figure alone. If this condition really is 

 met with, it must be very rare, for the author would otherwise have given a more 

 detailed account of it. The internal star of the second amphiaster is much less 

 developed and disappears earlier than in Limax. At its first appearance the male 

 pronucleus is situated just underneath the surface of the yolk, rarely in the 

 immediate vicinity of the nutritive pole, though more often in the nutritive 

 hemisphere. It has no relation with the protuberance at the nutritive pole. 

 He believes the latter is more accentuated when this pronucleus arises in the 

 formative hemisphere. Both pronuclei develop with the same rapidity and in 

 the same manner ; each soon presents a large nucleolus in its interior ; but the 

 female pronucleus advances little or not at all toward the centre of the vitellus, 

 because [?] it is soon joined by the male pronucleus, whose motion is infinitely 

 more rapid. After treatment with picric or acetic acid the pronuclei are, at 

 the moment of their appearance, homogeneous. A little later a certain number 

 of small spherical grains, each of which is furnished with a black point in its 

 centre, appear in the interior. Still later the pronuclei present the vesicular 

 character of true nuclei, the limit being formed by an irregular layer of vari- 

 able thickness. Neither osmic acid nor alcohol causes this layer to appear. 

 The contents remain clear and transparent after treatment with osmic acid, but 

 become granular with the other reagents mentioned. The nucleolus is variable 

 in different eggs. More often there is only a large one in each nucleus, but it 

 often happens that there are instejftl several small nucleoli. Since the latter 

 condition occurs in less advanced stages, it may be that the nucleoli become 

 fused, or that one is developed to the exclusion of the others. The rays which 

 surround the male pronucleus during its displacement, and are visible in the 

 living egg, disappear {^fter the use of reagents. It is possible that the same 

 may be the case with Limax. Fol has given no figures of this stellate arrange- 

 ment, but his statement is explicit. The two pronuclei may have attained 

 their full size at the time of contact, or " they may be still relatively little de- 

 veloped (Fig. 7), and in the latter case the conjugated nucleus will be obliged to 

 increase after its formation." I doubt if this last statement is warranted by the 

 figure cited. There does not appear to be here, more than with Limax, a veri- 

 table conjugation nucleus. Unless I misinterpret this figure, it shows already 

 the beginnings of the amphiaster of segmentation, and there cannot well be a 

 further growth, but only a metamorphosis of the two pronuclei into a segmen- 

 tation spindle. The nucleoli, continues Fol, still exist when the pronuclei are 

 in juxtaposition, but they disappear at the moment when the latter are fused. 

 During this fusion picric acid still causes the enveloping layer to appear, and, 

 within, granulations arranged in lines diverging from the point of union. 



The results of the fecundation of immature or over-ripe eggs of Asterias, or 

 such as are taken from animals kept in confinement, all being abnormal, have 

 been given at pp. 484, 485, 491. 



The phenomena of segmentation were most extensively pursued in Toxo- 



voL. VI. — NO. 12. 37 



