No. 2.] THE SEA-URCHIN EGG. 465 



center of the sperm-aster and of each of the daughter-asters 

 derived from it at first consists of a homogeneous mass (/. c. 

 p. 84) in which a reticular centrosphere is afterwards differen- 

 tiated. Within this centrosphere a centriole ("Enkelperiplast") 

 is subsequently formed, arising endogenously through "neue 

 Assimilation des aufgenommenen Materials " as a minute 

 central body (p. 85). This body then divides into two, each of 

 which ultimately enlarges to form a new reticular centrosphere 

 (I. c., p. 100) for the succeeding cleavage, and in this a new 

 centriole is again .endogenously formed, and so on. In other 

 words, the centriole (centrosome of Strasburger) has here no 

 persistent individual existence, but is formed de novo out of 

 the substance of the centrosphere (centrosome of Boveri) at 

 every cell-division, as in Toxopnenstes. The two forms differ, 

 however, in the fact that in Toxopnenstes the centriole does not 

 appear until after fission of the aster, whereas in RyncJielniis it 

 appears before fission, and leads the way in the fission of the 

 aster by its own division. ^ 



What, then, shall we identify in the sperm-aster of Toxo- 

 pnenstes as the "centrosome" in Boveri's sense — i.e., as the 

 structure that divides to form the dynamic centers of the 

 ensuing cleavage .'' I think that the only structure that 

 answers to this definition is the central mass of the aster — 

 that is, the substance of the original middle-piece — without 

 regard to its subsequent morphological differentiation. I 

 cannot deny the possibility that this body may contain from 

 the first a centriole (representing the true centrosome) too 



1 It would seem from Boveri's figures and descriptions that the behavior of 

 the centriole in Ascaris, is, in some respects, closely similar to what I have 

 described in Toxopnenstes. In the earlier stages of the archoplasm-masses no 

 centrosome was found (Zellenstudien, II, p. 751, Taf. XX, Figs. 27, 28). It then 

 appears as a small dark body in the center of the archoplasm (p. 752, Fig. 32), 

 divides into two (Fig. 35), followed by division of the archoplasm-sphere, enlarges, 

 and then for the first time a distinct "kleines centrales Korn," or centriole, 

 appears in the center (p. 754, Figs. 36-43). In the later stages of division the 

 centriole is no longer seen (Taf. XXII, Fig. 65-71), nor is it .shown in the 2-celled 

 stage until after the division of the centrosome and archoplasm-sphere (Figs. 74-77). 

 It would seem, however, that the relation between centriole and centrosome is 

 different in the two cases, though the centrosome is so small in Ascaris, and 

 Boveri's references to the centriole so brief, that their precise relation remains 

 in doubt. 



