MUSEUM OF COMPARA.TIVE ZOOLOGY. 523 



a result of the traction ; but taking into account Hertwig's figures and 

 what I have seen in Limax, I am inclined to think that all are due to 

 the same cause, and that the male aster centres, as Fol claims, in ad- 

 vance of the nucleus, and at least may induce its elongation. In that 

 event, the exact centre of radiation has not been clearly seen by Hertwig. 

 If so experienced an observer has overlooked the true relation of nucleus 

 and aster in so favorable a case as that of Sagitta, it will not be too much 

 to say that renewed observations directed especially to this point may 

 prove that the relation of aster to nucleus has hitherto been only par- 

 tially comprehended. 



Whether, however, the formation of asters can really be regarded as 

 the first visible alteration in the nuclear metamorphosis, may still be 

 open to question. There are many descriptions of important changes 

 occurring in the nucleus prior to the detection of any stellate figure. 

 Especially in the metamorphosis of the germinative vesicle is this the 

 case. But, from one cause or another, most of these descriptions can- 

 not be considered as definitely excluding the possibility that stellar fig- 

 ures accompany or precede the indicated changes. The very careful 

 account of the metamorphosis of the germinative vesicle in Asteracan- 

 thion given by Van Beneden, for example, affords no means of deciding 

 this question, since he failed to discover the asters at any stage. The 

 necessity in most cases of using reagents to demonstrate the stellar rays, 

 makes all continuous observations on living specimens of little or no 

 value in endeavoring to ascertain the synchronism of the astral phenom- 

 ena and the changes within the nucleus. Were the first detection of 

 rays in living eggs equivalent to finding the very beginning of such 

 structures, the question would have been long ago definitely answered 

 by Auerbach's studies, for he recorded the disappearance of the nucleoli 

 at the time of the confluence of the two pronuclei, and observed the 

 stellate figures only at a later period. But the timely use of reagents 

 would probably have shown the existence of asters at stages as early as 

 those in which they are found in Limax. 



A possible objection to the view that stars introduce the nuclear meta- 

 morphosis is presented in the division of tissue cells, where astral figures 

 of the protoplasm are less pronounced or altogether invisible. Here the 

 relatively great size of the nucleus, and the prominence of its labyrinthine 

 filaments are such as to make a study of the radial appearances in the 

 cell protoplasm much more difficult and unsatisfactory than in early em- 

 bryonic cells. But even here, according to Flemming's studies on the 

 epithelium of the salamander, centres of attraction are found at the poles 



