No. I.] THE EMBRYOLOGY OF CREPIDULA. 41 



and touch each other only by a comparatively small surface ; 

 later they become much more closely pressed together, and the 

 surface by which they are in contact becomes much larger, so 

 that each of the blastomeres is almost a hemisphere. Fig. 7. 



Immediately after the division of the nucleus the archo- 

 plasmic bodies, Figs. 5 and 6, begin to increase in size and to 

 become much more definite in outline. Each one lies close 

 beside the nucleus in the position of the pole of the preceding 

 spindle, and in surface preparations looks as if it might be the 

 shadow of the nucleus. My attention was first called to these 

 bodies by finding what I supposed to be two nuclei in each 

 cell, one of which was fainter in color and outline than the 

 other and looked as if it might be at a lower level in the &^^, 

 and it was some time before I could bring myself to believe that 

 these bodies, which are so plainly visible even in preparations 

 of the whole &^,^, and which in many cases are fully as large as 

 the nuclei themselves, were nothing other than the " archo- 

 plasmic bodies" of Boveri or the " spheres attractive" of van 

 Beneden.^ 



A careful study of one of these bodies in its resting stage 

 shows that it is a clear vesicular structure, containing appar- 

 ently a finely granular fluid and having a fairly definite outline 

 from which radiations proceed in every direction, Figs. 5, 6, 8, 

 10, etc. As it begins to divide, however, preparatory to the 

 formation of the karyokinetic spindle, the definite outline of 

 the body grows fainter and fainter until it cannot be recognized, 

 while the radiations extend much further through the proto- 

 plasm of the cell. 



At the close of the first cleavage, the nuclei, asters, and pro- 

 toplasmic areas lie directly opposite each other in the two 

 blastomeres, Fig. 5, but as soon as the blastomeres begin to 

 flatten against each other and the whole &gg assumes a more 

 compact form all these structures move in the direction of a 

 clock's hands, as shown in Fig. 6. This viovemeiit of the nuclei, 

 asters, and pTOtoplasm takes place invariably in the same direc- 



1 1 shall throughout this paper call these bodies the asters, a. name first used in 

 this connection by Fol ('73) to signify the radiating cytoplasmic structure within 

 the cell. 



