MUSEUM OF COMPARATIVE ZOOLOGY. 287 



BuTSCHLi ('75, pp. 210-213) is impelled in his studies on Nematodes 

 and snails to admit his former notion (that the nucleus simply length- 

 ens and divides) to be untenable ; but is not willing to follow Auerbach 

 in concluding that the nuclear substance is distributed through the pro- 

 toplasm. As regards the nuclei, they are, however, as Auerbach main- 

 tained, neiv structures. The most interesting part of Biitschli's discovery 

 is the spindle-shaped body (p. 208) an account of which is given in an- 

 other connection (see p. 536). Of the spindle which occupies the place 

 of the primary cleavage nucleus after the latter has assumed an unrec- 

 ognizable state, Biitschli says (pp. 211, 212) it greatly resembles the 

 spindle just described. In the earliest stages of its visibility there lies 

 a dark lustrous granule in each fibre at the equator of the body, so that 

 in a view upon the end of the spindle the granules together form a circle. 

 Changes, similar to those which occur in the division of the infusorian 

 "semen-capsule," now take place. There arise, namely, out of the 

 simple circle of granules two circles, which move apart toward the ends 

 of the spindle until they have finally arrived near the middle points of 

 the future cleavage spheres ; then the pointed ends of the spindle are 

 usually no longer visible, and one sees only the two circles of granules 

 and the fibres uniting them. Meanwhile the cleavage is nearly com- 

 pleted. When the formation of the nuclei begins, every distinct trace 

 of the circles and fibres has disappeared, but what has become of them 

 he does not know. Reasoning from the first-described spindle and its 

 supposed origin from the equivalent of the infusiorian " nucleolus " (the 

 germinative spot), Biitschli concludes that this spindle must owe its 

 origin to the nucleolus of the primary cleavage sphere, although he was 

 unable to recognize this nucleolus at any time previous to the appear- 

 ance of the spindle. 



Biitschli also observed that the new nuclei, even in the later genera- 

 tions, arise, as does the nucleus of the primary cleavage sphere, by the 

 fusing of numerous nuclei which first grow from minute beginnings to a 

 considerable size, and which in Cucullanus elegans arise at widely sepa- 

 rated points. Likewise in Lymnseus auricularis there "arise eight or 

 more small, vesicular, very clear nuclei, containing a number of dark 

 corpuscles, which are not to be taken for nucleoli." These nuclei sub- 

 sequently grow and successively unite to form a single nucleus,* — 



* Oellacher ('72^ pp. 406-416, Taf. XXXIII. Figs. 29-36) had already observed 

 that the nucleus of segmentation spheres in the trout was composed of a cluster (as 

 many as a dozen) of round or oval bodies varying somewhat in size, and containing 

 each — at least during the first stages of cleavage — a single nucleolus. Oellacher 



