CLEAVAGE AND DIFFERENTIATION 



99 



B 



Fig. 45 

 Mosaic development in the Ascidian Styela. (Compare fig. 59.) Two blasto- 

 meres of the 4-cell stage have been killed in each case. In A and B, these 

 are the two left blastomeres, resulting in the formation of (A) a right half- 

 gastrula, (B) a right half-larva with one muscle rudiment {ms.) and mesenchyme- 

 rudiment (tnch.). In C and D, the two posterior blastomeres have been killed, 

 leading to the formation of anterior half-embryos with complete neural plate but 

 no muscles. In E and F, the two anterior blastomeres have been killed. E, The 

 segmentation is typical of a posterior half. F, A posterior half-embryo is pro- 

 duced with complete muscle-rudiments but no neural plate or notochord. The 

 only regulation is the overgrowth of the ectoderm and the form-regulation of the 

 notochord. (From Conklin, Chap, ix of Cowdry, General Cytology, Chicago, 

 1924.) 



7-2 



