244 bulletin: museum of comparative zoology. 



The equatorial band as a whole has now moved to a position distinctly 

 within the margin of the dorsal surface (Fig. 71), so that a row of cells 

 from the ectodermal group of the ventral hemisphere has come into view 

 outside it round almost the entire periphery of the embryo (cf. Fig. 62). 

 This change has come about in consequence of additional divisions in the 

 ectodermal group of cells, which now not only has spread over the entire 

 ventral surface of the embryo,, but is encroaching upon its dorsal surface. 



Division has occurred nearly synchronously in all the cells of the ecto- 

 dermal group, though somewhat sooner intiiose nearest the animal pole. 

 (See Plate X. Figs. 63-70.) The strongest possible confirmation of my 

 own observation regarding the simultaneousness of division in the cells 

 of the ectodermal group in this period of development is afforded by 

 Satnassa's ('94) Figures 10 and 11, Taf. II. These represent respectively 

 a dorsal and a lateral view of a stage intermediate between those shown in 

 my Figures 62 and 71. In Samassa's Figures 10 and 11, all the cells of 

 what I have called the equatorial band are figured as containing qui- 

 escent nuclei, except the four seen to be mitotic in my Figure 62 

 (Samassa's cells 3 and 6, Fig. 10). The other cells of the ventral 

 hemisphere visible in Samassa's figures are without exception in process 

 of division. It may accordingly be confidently assumed that at the stage 

 shown in Figure 71 the cells of the ectodermal group of the ventral 

 hemisphere have all passed into the ninth generation. If so, they number 

 sixty-four ; this agrees well with the approximate count which one can 

 make from dorsal and ventral views, though it is impossible to be sure 

 about the exact number of ectoderm cells lying at this stage in an equa- 

 torial position between the dorsal and ventral surfaces. Accordingly I 

 shall not attempt to give for this and subsequent stages the lineage of 

 the individual cells of the ectodermal group. This would be a work of 

 great difficulty and of some uncertainty, for in this case the cells entirely 

 lack those marked differences of size, stainability, and arrangement 

 which make the lineage for the cells of the equatorial band and dorsal 

 hemisphere a matter of perfect definiteness. 



If the estimate given of the number of cells in the ectodermal group 

 is correct, the embryo shown in Plate XL Fig. 71 represents a stage of 

 one hundred and twelve cells distributed as follows. 



Ventral hemisphere : — 



64 cells in the 9th generation = the ectodermal group, 



20 " 8th " ) . . . , 



u 7th tt - = the equatorial band. 



90 — 



