520 BULLETIN OF THE 



These two asters are, however, in so many points alike, that one is war- 

 ranted in considering them, for the present at least, as the results of like 

 processes. The relation which the centre of the aster sustains to the 

 growing nucleus in cell division cannot be urged as the basis of even a 

 topographical difference, for Fol has recently reported a condition in the 

 case of the male pronucleus of Sagitta, which shows conclusively that the 

 centre of the male aster does not necessarily coincide with the centre of 

 its nuclear structure, any more than new nuclei are coincident with astral 

 centres in division. 



The most detailed, and, so far as one can judge without personal ob- 

 servation of the same object, the most accurate description of the changes 

 introducing the nuclear metamorphosis is that given by 0. Hertwig for 

 the germinative vesicle of Asteracanthion. The first changes are ob- 

 served in the protoplasm which surrounds the vesicle. The protuberance 

 of protoplasm which invades the vesicle has a clear spot near its apex 

 free from granules, and it sends out long protoplasmic ridges which en- 

 croach upon the vesicle. Although he intimates that the first small aster 



of the female aster. There is, however, another instance, Sagitta, in which the fe- 

 male pronucleus is represented as occupying the centre of an extensive radial system 

 (0. Hertwig, '78«, Taf. X. Fig. 11), Fol, however, makes no mention of such a sys- 

 tem, which seems the more remarkable as he observed the peculiar condition of the 

 male aster in Sagitta. 



Another radial figure, that which surrounds uniformly the conjugated pronuclei, 

 may also possibly be a separate phase of the astral phenomenon. For the present, 

 however, I believe it may safely be regarded as a continuation, and perhaps an exten- 

 sion of the so-called male aster. 0. Hertwig ('75) has described it in Toxopneustes 

 (pp. 400, 401) as though it might be genetically connected with the two asters which 

 arise at the first segmentation, as well as with the male aster, but in his general con- 

 clusions (p. 416) he has very definitely stated that this old single nucleus is dissolved, 

 and that the asters of segmentation arise as new structures. Hertwig and Selenka 

 agree in making the male pronucleus much smaller than the female when they come 

 in contact, and Selenka has recently come to the conclusion that the former continues 

 to increase in size until it equals the latter before there is a real fusion of their sub- 

 stances. May it not be that the extensive radial system surrounding welded but un- 

 fused pronuclei is only a male aster which ceases to exist when its nucleus has attained 

 normal dimensions ? It is possible that the aster of the female pronucleus, when such 

 exists, shares in the production of this central sun. The entire absence of both male 

 and female asters in the case of Limax might perhaps in that event be a sufficient ex- 

 planation of the non-appearance of a conjugation aster ; hut it cannot be denied that 

 the fusion of male and female pronuclei might also generate a force capable of induc- 

 ing similar radiations, for there is reason to believe that their substances are suffi- 

 ciently unlike to exert a mutual attraction. 



But if either of these asters is constantly developed, it remains yet to discover the 

 means of making their rays visible. 



