334 PROCEEDINGS OF THE ACADEMY OF [Julv, 



as, for example, in the relative development of the "cross" in the two 

 forms, are only such as frequently exist between other genera in which 

 the eggs are about equal in size. In short the enormous size of the 

 Fulgur egg, which has so greatly altered the relative proportions of 

 cytoplasm and yolk, and of micromeres and macromeres, has not 

 modified in the least the pattern of the cleavage. It has not 

 modified the localization in the egg and the segregation in 

 the cleavage of the substances of the germinal layers. In all of 

 these respects the egg of Fulgur is tj'pically like that of other gastero- 

 pods, and it shows no approach whatever to the condition found in the 

 eggs of cephalopods. 



7. RELATIVE SIZES OF NUCLEI AND CELLS. 



The great quantity of yolk in the egg of Fulgur introduces some 

 interesting cytological conditions. I have already called attention (p. 

 325) to the extremely small nuclear sjiindle in the first cleavage, and 

 to the problem of how this little spindle and the small amount of 

 cytoplasm around it can bring about the division of so great a cell 

 body. Another, though related, problem is found in the relative sizes 

 of the nucleus and cell body at different periods of the cleavage. 



In all the early cleavages the nuclei and spindles are very small, 

 even in the macromeres (figs. 1-6). Indeed the nuclei and spindles 

 in the macromeres are not larger than those in the micromeres during 

 the formation of the three quartets. After the formation of the third 

 quartet (fig. 7) there is a relatively long resting period for all of the 

 macromeres except D, consequently the nuclei in A, B, and C, fig. 8, 

 have grown to nearly double the diameter of those in fig. 7. The 

 macromere D divides before its nucleus becomes very large, giving 

 rise to the mesentomere, 4d. About the 50-cell stage, fig. 9, the macro- 

 meres A, B, and C divide, giving rise to the secondary macromeres 4A, 

 4B, and 4C The mitotic figures for this division are cpiite large and 

 are proportional in size to the resting nuclei from which they arise. 



From this stage onward the nuclei of all the macromeres enjoy a 

 long "rest," during which they grow to double their rnaxinuun 

 diameter at any previous stage (figs. 9-24). The greatest diameter of 

 the nucleus during the 2-cell and 4-cell stages is about 40 //; during the 

 formation of the three quartets the nuclear diameter is not greater 

 than this, but rather smaller; before the formation of the secondary 

 macromeres (fig. 8) the diameter of the nuclei in A, B, and C is about 

 50 //; at the close of their long rest and just before their next division 

 (fig. 24) the nuclei are about 80 ix in diameter. 



