284 



SCIENCE. 



[N. S. Vol. XXI. No. 530. 



paratively recently it was not suspected 

 that the cells thus formed in the earliest 

 stages had any constant and definite rela- 

 tion to the parts of the future body. The 

 fact has how been established, however, 



instance, the first cleavage-furrow passes 

 pretty accurately through the future me- 

 dian plane of the. body, and the two cells 

 thus formed give rise respectively to the 

 right and left sides of the embryo. In a 



Fig. 1. — Development of entire eggs and of isolated blastomeres of two-cell stage. A, 

 Dentalium ; at the left, development of the whole egg; at the right, development of the iso- 

 lated first two cells, producing two defective larvae. B, Amphioxus; the corresponding ey- 

 periment, isolated cells producing two perfect dwarfs. 



that in a large number of forms (though 

 apparently not in all) such a definite rela- 

 tion exists, both the form of division and 

 the prospective values of the cells being 

 constant. In the egg of the ascidian, for 



snail's egg the relation is a difit'erent one, 

 but is no less definite and constant; in the 

 four-cell stage, for instance, the material 

 that will produce the shell and foot is lo- 

 cated, mainly at least, in one of the four 



