314 



Charles Zeleny. 



cell stage passes through a definite seven-cell (28/4) stage. Such 

 a stage is represented in Figures 14B and C. 



Fig. 14 (x 216). 



Cleavage Stages and Larva from Two-Cell Groups and Isolated Blastomcres of the 



Four-Cell Stage. 



A, seven-cell stage from fragment (two cells+) of tgg. B, seven-cell 

 stage from isolated blastomere of an egg. C, seven-cell stage from other 

 isolated blastomere of egg shown in B. D, larva (ager=33 hours) from two- 

 cell group. Note apical plate, solid enteric invagination and large balstoccele 

 with numerous free cells. E, larva (age=r33 hours) from isolated blastomere. 

 Note the very large solid enteron nearly filling the blastocoele. 



Ten eggs were used for the experiments on the localization of 

 the morphogenic factors. The only larva developed from a two- 

 cell fragment was asymmetrical and swam in a small circle. 

 Thirty-three hours after fertilization it had an apical plate and 

 cilia, the beginning of the ingrowth of the archenteric mass and a 

 large blastocoele containing rounded and irregular mesenchyme 

 cells (Fig. 14D). There is thus no definite specification of the 

 morphogenic factors in a two-cell group of the four-cell stage. 



In six cases the four blastomeres were isolated, and in five of 

 them very rapidly rotating larvae resulted. The observations on 

 these were made in most cases thirty-three hours after the fertili- 



