190 Biology in America 



case they are present in both sperm and egg cell ; in the latter 

 case only in the egg, the amount of cytoplasm in the typical 

 sperm being too small to contain the "organ-forming sub- 

 stances. " 



If such formative stuffs are unequally distributed to dif- 

 ferent daughter cells in the division of the egg, then we should 

 expect each of these cells to give rise to a definite part of the 

 embryo and to that part only. If, on the other hand, these 





T ,:., 



y . 



"^ 



, :/- v .j; 



G * 



(Left) THE EGG OF THE TUNICATE CYNTHIA 



Showing the " organ forming substances" and their distribution in 

 different stages, a, anterior; p, posterior pole of egg; c, clear proto- 

 plasm; cr, yellow crescent; e, cortex containing yellow pigment; g.v., 

 germinal vesicle; k, chorion; p. b., polar bodies; t, test cells; y, yolk; 

 y. h., yellow hemisphere; 6, sperm nucleus. From Kellicott, after Wilson. 



(Eight) DEVELOPMENT OF THE MOLLUSC DENTALIUM 

 A, distribution of materials in undivided egg; B, commencement of 

 division showing the "polar lobe" p, which in C and D (division stages) 

 is found at D and X respectively. In E the cell X is absent, the polar 

 lobe having been removed at an earlier stage. F and H, normal larvae 

 of twenty-four and seventy-two hours, respectively, G and I, larvae of 

 the same ages lacking the " polar lobe" material. From Kellicott after 

 Wilson. 



stuffs are equally distributed to the daughter cells, then these 

 cells should be mutually interchangeable, and any one of 

 them, if isolated from its fellows, should give rise to com- 

 plete, though dwarfed embryos. Is the egg a mosaic, or is 

 it uniform in its structure? 



The ctenophore Beroe has normally eight rows of ciliary 

 bands. After one division of the egg, if the two resulting 



