MUSEUM OF COMPARATIVE ZOOLOGY. 435 



occupies this depression in the surface of the yolk, when only a trace of 

 the germinative vesicle is to be seen {loc. cit.. Fig. 13). The failure to 

 make use of proper reagents is sufficient to explain the absence of every 

 thing relating to the internal appearance of the e^g at this time. 



ScHENK ('77j claims for the germinative dot of Serpula nearly the 

 same function which was ascribed by F. Miiller to the " Richtungsblas- 

 chen." On a former occasion he did not find polar globules, but instead 

 a flattened body which became pressed into the yolk and ceased to be 

 visible. He now maintains that this structure can be considered the 

 expelled germinative dot, for after fecundation one can follow it from its 

 existence within the germinative vesicle until its complete emergence 

 from the Qgg. After its exit, it is at first round, and only later becomes 

 flat ; or on the same egg there may be an alternation of these forms. 

 The appearance of this corpuscle is followed by the well-known retrac- 

 tion of the vitellus and the appearance of a radial striation in the pro- 

 toplasm of the egg. This eliminated dot exercises in part the function 

 of polar globule ; it exerts a mechanical or other influence over the 

 yolk which leads to the production of furrows, — an impulse to cleav- 

 age. It is difficult to say whether the dot communicates this impulse 

 through some " Impression in den Dotter," or whether some other 

 stimulus is present. 



The grounds urged for this opinion do not appear to me of great 

 moment. The elimination of the corpuscle at the point of the surface 

 where the first furrow is soon to appear, and the existence of the cor- 

 puscle in the furrow when the latter does appear, are sufficient to show 

 that accurate observations will make of this corpuscle a polar globule, 

 but not sufficient to give support to the theory here propounded. 



In a note on fecundation Fol ('77) takes the position that there are 

 two well-marked cases in early stages of development : in one, which is 

 exemplified by the sea-urchin, there is a complete absence of the " cor- 

 puscule de rebut," the oviile at the moment of extrusion being already 

 destitute of germinative vesicle and possessing only a female pronucleus ; 

 in the other case, embracing most other animals, the vesicle is replaced 

 by a double stellate figure, one of the stars escapes to form the first 

 polar globule, and the second polar globule may be formed by a division 

 of the first, or, more often, like the first, by the formation of a second 

 double star. The substance thus expelled corresponds to the major 

 part of the germinative vesicle enveloped by a little vitelline protoplasm. 

 The principal difierence in these two cases consists in the epoch of the 

 disappearance of the germinative vesicle, whether precocious or tardy. 



