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, 



i ch. 



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 blastomeres. 

 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. D. 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, irich, mesenchyme; ms, muscle cells; 

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



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

 some time and forms those parts, and only those, which it 

 would have formed, had development of the entire cell group 



