2 94 Charles Zeleny. 



gestlon of Professor Wilson/ The special aim of the experi- 

 ments was two-fold. In the first place it was desired to throw 

 some light upon the character of changes in localization which 

 take place between the time of fertilization of the egg and the 

 completion of the first cleavage. Since a fragment of the unfer- 

 tilized egg segments as a whole while an isolated blastomere of 

 the two-cell stage segments as a half, fragments of the egg taken 

 at intermediate stages must yield interesting results. In the sec- 

 ond place a comparative study was made of the characteristics ex- 

 hibited by larvae developed from different portions of the egg 

 isolated at the eight-cell stage. The three portions thus compared 

 are the upper and the lower quartets obtained by a horizontal cut 

 and the lateral four-cell groups obtained by a vertical cut. 



Clear results were obtained on these two points. For the first 

 it is shown that there is a progressive localization of the cleavage 

 factors between the time of fertilization and the completion of the 

 first cleavage. For the second a definite differentiation along the 

 polar axis of the egg is made out at the eight-cell stage. This 

 differentiation occurs in such a way that while a lateral four-cell 

 group remains totipotent, the upper and lower quartets are no 

 longer so, one lacking the ability to form an enteron and the other 

 the ability to form an apical organ. 



2. Method. 



The method of operation was a very simple one. The eggs 

 were placed on a glass slide and the water was withdrawn until 

 they were slightly flattened. The cut was made with a fine-bladed 

 scalpel under a dissecting microscope. The resulting parts were 

 then placed in individual dishes, where they were allowed to de- 

 velop. In fragments of the undivided egg the segments of the 

 sphere thus obtained retained their shape for several minutes, but 

 gradually assumed the spherical form. Fragments of unfertil- 

 ized eggs were fertilized after the spherical shape had been as- 



il wish to express my great obligation to Professor Wilson for invaluable 

 advice during the progress of the experiments, and to the members of the 

 staflf at the Naples Zoological Station for continued kindnesses. 



