MUSEUM OF COMPARATIVE ZOOLOGY. 399 



changes in the Pteropod egg. ''At the moment of deposit one only sees 

 in the midst of the protoplasm two molecular stars." After the escape 

 of the polar globules, there appears a nuclear structure (which Fol still 

 insists upon calling a germinative vesicle), which in turn soon disappears, 

 giving place to two molecular stars. This is the beginning of the seg- 

 mentation, as a,lready described for Geryonia. " I would only add," he 

 says, " that I have seen these stars arise in the interior of the germina- 

 tive vesicle, an instant before its diappearance." 



The first change in the germinative vesicle of fertilized eggs of Serpula 

 uncinata consists, according to Schenk (74^, pp. 291-294, Figs. 3-8), 

 in its becoming notched. This change is effected by a motion in the 

 granular protoplasm of the yolk, which is directed, as larger and smaller 

 processes, toward the centre of the vesicle. During many passive altera- 

 tions of form the vesicle becomes smaller, and at length reaches the sur- 

 face of the yolk. Here it is for a time distinguishable as a clear space, 

 but finally this fades away till no recognizable trace of it is left. Mean- 

 while the germinative dot is eliminated, and lies between the yolk and 

 its envelope, the latter being raised into a corresponding prominence. 

 •This eliminated dot, which is plainly stained in carmine, becomes flat- 

 tened against the yolk, and finally ceases to be visible, the envelope 

 assuming its full circular outline. The changes of the vesicle, but not 

 the elimination of the dot, were also seen in unfecundated eggs. 



Schenk adds, that one cannot be easily induced to maintain for this 

 structure such a role as Robin ascribes to his polar globules, since in this 

 case the fate of the germinative dot cannot be further followed, and that 

 he has been unable to observe polar globules either in the case of Ser- 

 pula or Phallusia intestinalis. 



A sudden contraction of the yolk follows, and afterwards it again fills 

 the membrane completely. The appearance of a stellate figure follows, 

 as described at page 283. 



Although principally occupied with the events which succeed matura- 

 tion, AuERBACH has contributed much to the understanding of this sub- 

 ject, for he, more than any one else, has fixed the attention of embryolo- 

 gists upon the nature and origin of the first cleavage nucleus, — upon 

 the existence of two nuclear structures which, with Ed. van Beneden, 

 1 have designated "pronuclei." Auerbach ('74, pp. 195 et seq., Figs. 

 1-7) begins the account of his studies on Ascaris and Strongylus with 

 a stage which follows very promptly on the fecundation of the egg. By 

 this epoch of fecundation, however, we are simply to understand a point 

 of time at which the spermatozoa are supposed to come in contact with 



