CHEMODIFFERENTIATION 



59 



Several experiments have proved that the cells containing 

 the different types of protoplasm really do have different 

 developmental potencies. Isolated blastomeres produce almost 

 exclusively structures to which they would have given rise 

 in normal development (Conklin, 1905-11). If all the cells con- 

 taining a particular type of protoplasm are removed, the larva 

 will lack the organs concerned (Von Ubisch, 1939). Centrifuga- 

 tion modifies the relative positions of the protoplasmic sub- 

 stances in the uncleaved egg ; such treatment results in embryos 

 in which the various tissues and organs are present, but mixed 



<?c^r. _ 



^'cjcV^^: 



Fig. 23. (a) an ascidian larva, seen from the right, after Kowa- 

 lewsky; (b-c) abnormal embryos formed by centrifuged eggs, after 

 Conklin. ect =■ ectoderm, ent = endoderm, mes = mesoderm, tnch^=^ 

 mesenchyme, zw = nervous tissue, ch = notochord, hh =^ brain 

 vesicle, st = statocyst, o ^ eye spot. 



