324 Charles Zeleny. 



as the materials In the Isolated blastomere of the frog's egg are 

 undoubtedly specialized so as to form a half cleavage pattern and 

 a half embryo under the ordinary conditions, the re-adjustment of 

 materials due to Inversion gives the necessary conditions for regu- 

 lation. May not the unfertilized egg of Cerebratulus likewise 

 show a localization of formative factors so that a fragment is a 

 true portion of a mosaic, but needs only the conditions accompany- 

 ing the streaming during the maturation stages to accomplish a 

 readjustment to a whole? Evidently there Is no means of de- 

 termining this point, because If we assume the possibility of a re- 

 adjustment during the maturation stages, we remove our only hope 

 of a direct method of deciding the question as to the presence or 

 absence of developmental specification before fertilization. The 

 only remaining method lies In Indirect Inferences from the ob- 

 served localization of visible materials during these stages. There 

 Is abundant proof of such a progressive localization during this 

 time, and the conclusion that there is an arrangement of forma- 

 tive materials Into a definite pattern at this period Is a natural one. 

 For there can be no valid objection to the association of the two 

 parallel processes of localization of visible materials and of re- 

 sultant cleavage and morphogenic factors. 



But why Is there a progressive localization of the morpho- 

 genic factors In the unsegmented egg, as Indicated by Yatsu's 

 work, while at the same time the Isolated blastomere of the two- 

 celled stage develops Into a perfect larva ? The progressive local- 

 ization of cleavage factors as shown in my experiments Is naturally 

 to be expected, since there is a gradual passage from a whole 

 cleavage on the one hand to a half cleavage on the other. We 

 may assume a gradual localization of materials controlling these 

 factors, or a greater opportunity for regulation In the earlier as 

 compared with the later stages,' or both, to account for the data. 

 With the progressive localization of morphogenic factors, as de- 

 scribed by Yatsu, there Is no such sequence. Starting with com- 

 plete larvae developing from the fragments of unfertilized eggs, 

 there is a gradual increase in the defects in the larvae up to the 

 completion of the first cleavage. Then very suddenly, as soon as 

 the cleavage is completed, there Is a return to whole larvae. I 



