No. 2.] THE SEA-URCHIN EGG. 459 



Text-fig. VIII, B). The chromatic vesicles then fuse to- 

 gether, their number being progressively diminished until at 

 the time of division there are usually not more than two or 

 three at each side (Phototype 10, Text-fig. IX). As cleavage 

 takes place they finally unite completely to form the small 

 daughter-nuclei (Phototype 11, Text-fig. X, A), which are trav- 

 ersed by an exceedingly fine and delicate chromatic network, 

 and stain very slightly. In the ensuing pause or resting-stage 

 the nuclei rapidly grow (Phototype 12, Text-fig. X, E). The 

 chromatin resumes its staining capacity, and the succeeding 

 stages closely resemble the history of the cleavage-nucleus. 



III. The Centrosome. 



The terminology applied to the various portions of the aster 

 is so contradictory and confused that none of the usual terms 

 can safely be employed without clearly defining the exact sense 

 attached to them. This confusion arises mainly from the fact 

 that the word "centrosome " is at present used in three differ- 

 ent senses, being applied (i), by Boveri to the entire central 

 mass of the aster exclusive of the rays (" astrosphere " of Fol, 

 " centrosphere " of Strasburger), (2), by Strasburger and others 

 to a smaller dark body often found within the centrosphere, 

 and (3), by Heidenhain to the individual granules of which this 

 dark body is made up. Boveri, than whom certainly no one 

 can speak with a higher right, adopts in his latest paper (/. c) 

 the first of these meanings, and suggests the convenient word 

 " centriole " for the small dark bodies lying within the centro- 

 some of which they form a part. In view, however, of the 

 ambiguity of the word centrosome I shall not employ it in the 

 descriptive part of this paper, but shall adopt Strasburger's 

 term centrosphere (equivalent to Boveri's "centrosome" and 

 Fol's "astrosphere") for the spherical reticulated mass occu- 

 pying the center of the aster in the fully developed karyokinetic 

 figure. The word "centriole" will be applied to the minute 

 deeply staining body ("centrosome" of Strasburger) that in 

 some cases lies in the center of the centrosphere, in some cases 

 precedes its formation. (This body may apparently represent 



