5 go GRIFFIN. [Vol. XV. 



to the wall of the germinal vesicle, and about 45-90 degrees 

 from each other (PI. XXXI, Fig. 7). The rays of each aster 

 converge directly to a minute dark-staining granule, the centro- 

 some, which may at times be double even in this early stage. 

 I find no certain indication of a rupture of the nuclear membrane 

 behind the centrosomes, such as Mathews ('95) finds in Asterias, 

 indicating their possible nuclear origin. The slight flattening 

 or folding of the membrane seems to be rather the effect of 

 inwardly growing rays, since it becomes steadily more marked 

 as the asters develop. Not infrequently in these early stages 

 one aster seems a trifle more advanced than the other, since 

 the membrane behind it is considerably folded, while behind 

 the other it is only flattened or but slightly folded (PI. XXXI, 

 Fig. 7). This inequality soon disappears. 



In later stages the asters remain the same distance apart and 

 undergo no further divergence. This is quite at variance with 

 what occurs in most other forms. Platner ('89) {Aidastomiim), 

 Korschelt ('95) (Ophryotrocha), Wheeler ('95) [Myzostomtini), 

 Garnault ('88) (Helix), Mathews ('95) {Asterias), and others, all 

 describe diverging daughter-asters or centrosomes immediately 

 derived by division from a single one. It apparently agrees 

 with Thysanozoon (Van der Stricht, '94) and Chaetopterits (Mead, 

 97). The definitive spindle, in consequence, arises in all cases 

 secondarily by the meeting of rays from the opposite and inde- 

 pendent asters, as in Myzostoma (Wheeler), Salamander (Driiner, 

 '94), Selachians (Moore, '95), Opisthobranchs (MacFarland, '97), 

 and some other forms. 



Centrosomes. — In stages earlier than two to three minutes 

 after fertilization, a most careful search has failed to reveal any 

 structure that can, with certainty, be identified as a centrosome 

 or definitive aster. The unfertilized, and some of the early 

 fertilized eggs, however, exhibit an interesting phenomenon, 

 which, from its possible bearing upon certain facts recently 

 brought to light by experimental and other investigators (e.g., 

 Reinke, '94; Hertwig, '96; Mead, '97; Morgan, '96; Osterhout,'97; 

 Mottier, '97) well merits description. In preparations of both 

 full-grown and sometimes younger ova the strands of the cyto- 

 reticulum show a tendency to arrange themselves in radiating 



