Development of Rotifer vulgaris. ■ 137 



The following is what I have been able to ascertain by the 

 investigations of months (February to July 1884) upon the 

 development of Rotifer vulgaris. After its separation from 

 the ovary the egg shows a centrally-placed germinal vesicle 

 with a distinct germinal spot. The development of the embryo 

 commences by the contours of the germinal vesicle becoming- 

 indefinite, until it fades away and tinally becomes quite invi- 

 sible. When this has taken place the yelk-granules show a 

 tendency to accumulate in the middle of the egg } so that a 

 darker central mass and a lighter peripheral layer may be di- 

 stinguished. In from twenty to twenty-five minutes, however, 

 the egg-contents gradually clear again, and under suitable illu- 

 mination we discover that a division of the germinal vesicle has 

 taken place. The latter has certainly only a little diminished 

 in size, which no doubt is to be explained by its substance 

 having been replaced from the yelk. From the recent obser- 

 vations of Ludwig Will*, according to which masses of 

 nucleus-substance issue from the germinal vesicle and become 

 converted into yelk-substance, the reverse of this process is 

 also conceivable. The arrangement of the products of divi- 

 sion and the original germinal vesicle is usually such that the 

 latter are placed in the vicinity of one pole of the egg, and the 

 daughter vesicles in the region of the other pole. 



I have never been able to observe directly in the egg of 

 Rotifer the mode of segmentation ; but as the daughter vesicles 

 are always smaller than the original nucleus, it is probable 

 that the former proceed from the latter by a process of gem- 

 mation. Something like this, as is well known, occurs also 

 in the segmentation of the egg of Rhodites rosce, in which 

 (according to Weismann's observations) the so-called " pos- 

 terior pole-nucleus" likewise increases by gemmation f. 



It is worthy of note that the vitelline elements do not 

 immediately group themselves around the progeny of the 

 germinal vesicle, but there is rather a lapse of a certain time 

 (several hours !) before the commencement of a blastomere- 

 formation is to be recognized. So far as I know, this fact has 

 previously been indicated only by Leydig, who, as long since 

 as 1855, puts the following question with regard to it : — 

 " Are we to conclude that the germinal vesicle in the ovarian 

 egg has become converted directly by continual division into 



many nuclei or do the winter-eggs perhaps, even at 



their origin in the ovary, enclose a number of nuclei (germinal 



* Zool. Anzeiger, 1884, nos. 167, 168. 



t Beitrage zur Kenntuiss der ersten Vorgange iru Insektenoi, pp. 85 et 

 seqq. (1883). 



