322 Charles Zeleny. 



€ 



power of blastula fragments as compared with those of the eight- 

 cell and sixteen-cell stages. A similar fact was noted by Professor 

 Wilson for C. lacteus, and he supposes that a possibility of error 

 in orientation of the blastulae may account for the result. For this 

 reason I took special care in determining the orientation, and in 

 two of the three cases I think there is little doubt of the correct- 

 ness of the determination. 



5. Summary of results. 



1. Both nucleated and non-nucleated fragments of the unfer- 

 tilized eggs of Cerebratulus marginatus segment as wholes. 



2. Isolated blastomeres of the two-cell stage segment as if the 

 other blastomere were still in its place, i. e., they segment as ver- 

 tical halves. 



3. Fragments obtained during the stages between the fertili- 

 zation of the egg and the completion of the first cleavage show a 

 progressive specification of the cleavage factors as evidenced by 

 abnormalities in rhythm of division, size relations of cells and 

 position of cells. After the separation of the cleavage nuclei and 

 when the cytoplasm of the two cells is still widely connected, the 

 two halves when cut apart may already show all the characters of 

 half cleavages. 



4. Isolated blastomeres of the four-cell stage segment as 

 fourths of the whole cleavage pattern. 



5. Larvae developed from the upper quartet of the eight-cell 

 stage always possess an apical organ and lack an enteron, those 

 developed from the lower quartet always possess an enteron and 

 lack an apical organ, while those developed from lateral four- 

 cell groups containing two cells of the upper and two cells of the 

 lower quartet always possess both apical organ and enteron. 



6. Larvae developed from the upper four cells of the sixteen- 

 cell stage lack an enteron, but possess an apical organ and blasto- 

 coele. Those developed from the lower twelve cells have a large 

 enteron, but no apical organ or blastocoele. 



7. Two embryos developed by a secondary division from the 

 upper two-thirds of a blastula both developed apical organs. The 



