Observations and Experiments on the Ctenophore Egg. l * 



c) Experiment III (thirteen cases). 



Sections were made on the egg above the "cleavage head" at 

 various periods and along various planes, and the behavior of the- 

 enucleated fragments containing the "cleavage head" were studied. 



In eight cases out of thirteen the cleavage went on normally. 

 The rate of downward progress of the furrow was normal, 

 or a little slower than the normal that is 15/^ per minutes on 

 an average. It was sometimes 10// or even as slow as 8//- 

 per minutes. In Fig. 34 (PI. Ill) the section passed through 

 the middle of the cleavage furrow and in Fig. 39 (PL III) 

 the cut Avas made when the cleavage had just begun. In both the 

 enucleated pieces cleavage went on as though they were a part of 

 the Avhole egg. It is interesting to note that the movement of the- 

 ectoplasm is the same as in the entire egg, that is, a thick ecto- 

 plasmic accumulation is formed about the micromere pole towards 

 the end of the division (PI. Ill, Fig. 35), and after that thickenings 

 in the neighborhood of the macromere pole (PI. Ill, Figs. 36, 40). 

 Here one notices that the thickness of the above ectoplamic- 

 accumulation depends upon the size of the enucleated pieces. It 

 also may be remarked that similar up-and-down movement of the 

 ectoplasm takes place in the nucleated pieces (PI. Ill, Fig. 41). 



In studying carefully the relation between the angle of the cuts 

 and the direction of the cleavage furrow, the following results were 

 obtained. If the section be made when the cleavage furrow is 

 shallow, then the division goes on normally irrespective of the 

 angle of the cut (PI. Ill, Figs. 39. 40). If, on the other iiand, the 

 operation is performed in later stages, the cleavage is usually ac- 

 complished normally only when the section is horizontal or 

 approximately so ; if otherwise, the furrow is bent and the l)ending is 

 ahvays towards the side with more cytoplasm, so that tJie result- 

 ing blastomeres are of nearly the same size (PI. Ill, Figs. 37, 38). 



In three cases out of thirteen something unexpected hap- 

 pened. One of these cases is represented in Figs. 41 and 42 

 (PL III). The other two were very much hke this. The cleavage 

 furrow went down riear the micromere pole and turned upward so 



