No. 2.] THE SEA-URCHIN EGG. 463 



and, in some cases at any rate, actually surrounds them 

 completely. The ^^^ now divides, and the remains of the 

 centrosphere are now found as a clear substance staining pale 

 blue, that extends more or less completely around the daughter- 

 nucleus (Phototypes 10, 11, Text-figs. IX, X). The entire 

 aster now divides into two, the central mass of each daughter- 

 aster being formed of a nearly homogeneous or finely granular 

 substance, which I believe to be derived from the remains of 

 the centrosphere 1 (Phototype 12, Text-fig. X). The later history 

 of the aster repeats, step by step, the changes undergone by 

 the asters of the "pause," as described at p. 452. Precisely as 

 before, I can at first find no centriole in the central mass. 

 Later a group of minute granules appears, surrounded by a red 

 mass, and by the multiplication of these granules a reticular 

 centrosphere is formed as before. 



B. Conchtsions. — The foregoing observations show conclu- 

 sively, I think, that the reticular centrosphere (centrosome of 

 Boveri) of the aster arises by a morphological and chemical 

 differentiation of the central mass of the sperm-aster, and that 

 in its earliest stage it is represented by a very minute granule 

 or group of granules forming what would ordinarily be called 

 a "centrosome," but which I have called a centriole. Does this 

 centriole have the morphological value of a centrosome? — i.e., 

 is it, in Boveri's words, a distinct permanent cell-organ, that 

 multiplies by division and thus affords the dynamic centers for 

 the formation of the daughter-cells .-' If my observations are 

 correct, this question must be answered in the negative ; for 

 the centriole is not distinguishable, either in the original 

 middle-piece, in the undivided sperm-aster, or in the asters of 

 the early 2-cell stage. I am compelled therefore to conclude 

 that the centriole is formed endogenously in the central mass, 

 and that it is without morphological significance, being only an 

 expression of a secondary differentiation of the central mass 

 caused by unknown chemical and physical activities centering 



^ This account differs slightly from that given in my first paper (Journal of 

 Morphology, X, i, p. 325), since I had not at that time seen all the later stages, 

 and failed to observe the disappearance of the reticulated structure of the 

 centrosphere. 



