336 PROCEEDINGS OF THE ACADEMY OF [Julv, 



In the division of the cell IdocIv Fnlgur presents some interesting 

 conditicwis. Attention has been called to the fact that although the 

 spindle in the first cleavage is very small the entire egg divides. 

 Equally sin-prising is the fact that in the divisions of the primary and 

 secondary macromeres subsec|uent to the formation of the fourth 

 quartet, the nucleus only divides while the cell body remains imdivided 

 (figs. 17-28). The cell bodies of these entomeres do not divide again 

 until very late in development, if at all ; even in figs. 34 and 35, in 

 which the eml^ryo shows many definitive structures, the furrow be- 

 tween two of the original macromeres can be seen running ol)liquely 

 through the region below the head vesicle {H . V .). 



It seems to me that the following explanation may be offered of this 

 apparent anomaly. In the early stages of development the cell sub- 

 stance is quite fluid, as has been pointed out (p. 326), whereas in the 

 later stages the substance of the macromeres becomes much more con- 

 sistent. In the later stages of development the macromeres do not 

 show the tendency to burst or to undergo distortion during fixation, 

 which is so troublesome in the earlier stages: this is, I believe, due to 

 the greater consistency of the yolk during the later stages. This 

 same difference in consistency may explain also the curious fact that 

 the entire cell divides in the early stages, whereas only the nucleus 

 divides in later stages. 



In this connection attention may be called to the fact that there 

 is apparently a great increase in the ciuantity of cytoplasm in the later 

 stages, as compared with the earlier ones (cf. figs. 1, 6, 25). This 

 increased ciuantity of cytoplasm might be due to the liquefaction of 

 the yolk and its transformation into cytoplasm, or it might be the 

 result of the segregation into the ectodermal cap of cytoplasm originally 

 spread through the yolk substance. It is prol3a]:)le that both of these 

 processes occur, but in view of the increasing consistency of the yolk 

 during the period of quartet formation, it seems very probal^le that 

 most of the cytoplasm of the early stages was present as such, but in a 

 diffused form, in the unsegmented egg. 



III. Organogeny. 



It has been shoAvn that there are but few and minor differences- 

 between Fulgur and Crepidula in the cleavage up to a stage of about 

 100 cells (fig. 16). After this stage the differences in development 

 become more marked. The first notable difference is found in the 

 number of micromeres; by repeated divisions of the three quartets of 



