Observations and Experiments on the Ctenophoi'e Egg. «^ 



1)) Siihseiueiit Cleavajes. 



Soon alter the first cleavage is completed, the ectoplasmic 

 thickening near the micromere pole disappears and at the same 

 time an accumnlation comes in view over the macromere pole 

 (PI. II, Fig. 23). The second cleavage takes place in exactly the 

 same manner as the first (PI. II, Fig. 24). In the beginning of the 

 third cleavage an accumnlation of the ectoplasm near the macro- 

 mere pole is also seen (PI. II, Figs. 25, 26, 27). At the fourth 

 division the micromeres are formed, which are almost entirely 

 made up of the greenish ectoplasm (PL 11, Figs. 29, oO). Sub- 

 sequent divisions of the micromere are carried on in unilateral 

 fashion similar to the division of the entire egg. So also the 

 divisions of the macromeres (Text fig. II). 



Fig. II. 

 Two dividing entoderm cells of Beroë ovata. X60. 



V. Experiments on Cell-division.^ 



Fifty eggs of Beroc ovata were operated on during the first 

 «leavage in twelve different ways with the intention of testing, if 

 possible, hypotheses hetherto put forth regarding the division 

 mechanism of the ctenophore egg. On account of the large size 

 iind favorable consistency of the egg, the operations were performed 

 Avith the greatest ease by means of a small knife. Sometimes, 

 however, in case tlie jelly around the egg was unusually hard, 



I This part of the present p ip.^r was read as a preliminary note before the Seventh 

 International Zoological Congress at Boston 1907. 



