496 APPENDIX 



potential, due to the accumulation of ions of opposite sign at the 

 two poles, and resulting in the segregation of the egg-contents and 

 in their stratified re-arrangement. 



The next (third, equatorial) cleavage separates the animal cyto- 

 plasm (mostly clear, "alkaline") from the vegetative cytoplasm 

 (granular, "acid"): the former goes to the formation of the first 

 quartet of micromeres (i a to id), while the latter forms the 

 macromeres {i A to 1 D). The droplets which become included in 

 the micromeres are those which lay in the equatorial zone of the egg, 

 and, as we shall see, are destined to become included in the primary 

 trochoblasts (see also p. 132). 



The forces which were at work prior to the equatorial cleavage in 

 segregating the egg-contents, continue to function after that cleav- 

 age-division in both micromeres and macromeres. The result is that 

 in the micromeres, the albumen droplets which have become 

 included in these cells are concentrated at their most vegetative 

 end; while in the macromeres, what little clear cytoplasm there is, 

 is situated at their most animal end, the fat and albumen drops 

 being still further concentrated vegetatively. 



At the next (fourth) cleavage the droplets in the micromeres find 

 themselves in the primary trochoblasts (i <2^ to i d'^), while the 

 clear cytoplasm becomes included in the apical cells (1 a^ to 1 d^). 

 At the same time, the clear cytoplasm in the macromeres becomes 

 incorporated in the micromeres of the second quartet (2 a to 2 d), 

 while the fat drops, etc. remain in the macromeres (2 ^ to 2 D). 



In general, therefore, it appears that those regions of the cyto- 

 plasm which are characterised by an alkaline reaction give rise to 

 the ectoderm, while those regions with an acid reaction become 

 endoderm. Facts such as these throw an interesting light on the 

 inhibition of ectoderm in lithium-induced exogastrulae of Echinoids 

 (see p. 336). 



The free-swimming trochophore larva provides evidence of a 

 ventro-dorsal gradient, for the ventrally-situated first (ectodermal) 

 and second (mesodermal) somatoblasts are particularly alkaline, 

 while, in the endoderm, a similar gradient from ventral to dorsal 

 side may be observed. 



We do not yet know whether any of the various egg-contents 

 which we have seen are distributed to various blastomeres during 



