Observations and Experiments on the Ctenophore Egg. o 



«centric differentiations; namely (a) the extremely thin homogene- 

 ous outer layer, (b) the ectoplasm and (c) the entoplasm. 



The outer "membrane" is a very thin semifluid layer free 



from granules. It can hardly l)e called membrane. It is difficult, 



if not impossible, to detect it. When the egg is compressed or 



wherever accumulation of the ectoplasm takes place, one can see it 



fairly well. I should not hesitate to homologize this with the 



•ectosarc described by Andrews in the egg of Hydra ('98) and with 



a similar layer found in the sea-urchin egg, to Avhich Zjeglee, has 



.attached so much importance ('03, '04). It can not be looked 



upon as identical with the hypothetical cell-membrane of 



Rhumbler ('99). What rule this thin layer actually plays during 



•cleavage is problematical. I am rather inclined to think that it 



has very little to do with that process. 



The ectoplasm is, as has been described by many authors, a 

 rrather thick layer of finely alveolar plasm. It is of uniform thick- 

 ness until fertilized. This layer is, contrary to Zieoler' s view, not 

 .at all of the same nature as the "ectoplasm" of the sea-urchin egg. 

 It is fluorescent and looks green under reflected light, reminding 

 •one of a piece of uramium glass (cf. Chun '80 p. 100). In stimulating 

 the egg with a weak electric current this layer alone seems to emit 

 a beautiful greenish light. It should be mentioned, that, as the 

 -development advances, the ectoderm comes to monopolize this 

 property. In a Wood's Holl species, Mncmiopsis, the egg before 

 -cleavage, according to Peters ('05), was not phosphorescent. 



It need hardly be mentioned that the entoplasm is a coarse 

 .alveolor structure. The alveoles of the Callianira egg are much 

 fewer in number than those of the Beroë egg, the individual alveole 

 ■ of the former being much larger than that of the latter (PI. II, 

 I^igs. 27 and 28). On crushing the Callianira egg I could count 

 in one case 67, in another 64, and in still another 54 alveoles. 

 Ziegler calls the alveoler substance " transparent yolk granules " 

 ;('98 p. 36). This seems to have given Rhumbler tlie impression 

 îthat it was made up of a rather highly viscous substance. But 

 .as::a matter of fact, this is not so. Chun ('80) has called it " Zell- 

 rsaft." Though this term is not happily chosen, yet it is far better 



