272 



GENERAL EMBRYOLOGY 



and many others to an analysis of this problem of localization. 

 The blastomeres of many Echinoderms, Molluscs, etc., can be 

 thus separated, and it is a remarkable fact that one of two, four, 



ms. 



v.end/ 



FIG. 125. The development of blastomeres of the four-cell stage of Cynthia- 

 From Conklin. A. Anterior half-embryo derived from two anterior blastemeres- 

 The yellow crescent remains visible in the posterior, uninjured cells (B 3 ). Sense 

 spots are present but the neural plate never forms a tube. The chorda cells lie 

 in a heap at the left side. There is no trace of muscle substance or of a tail. 

 B. Posterior half-embryo from the two posterior blastomeres. Dorsal view, 

 focussed deeply upon the double row of ventral endoderm cells in the mid-line, 

 a mass of mesenchyme cells on each side. No neural or chorda cells. C. Left 

 anterior quarter embryo from cell A; dorsal view. An invagination of the 

 ectoderm cells has the appearance of a gastrula, but is probably the invagination 

 of the neural plate. >. Left anterior, and right posterior quarter-embryos, 

 from cells A and B; dorsal view. The former shows thickened ectoderm cells, 

 probably neural plate, around the endoderm cells; in the latter are eight muscle 

 cells and three caudal endoderm cells, m'ch, mesenchyme; ms, muscle cells; 

 n.p., neural plate; v.end., ventral endoderm. 



eight, or even one of sixteen cells, continues to develop for 

 ^ome time and forms those parts, and only those, which it 

 would have formed, had development of the entire cell group 



