Observations and Experiments on the Ctenophoro Eu'< 



1Ü 



^ucli cell division. It is also interesting to note, as I have done 

 elsewhere, that unilateral cleavage is seen in some parthenogeneti- 

 cally developing sea-urchin eggs and also in lamprey eggs, whose 

 ooplasm is uniform ely laden with yolk granules. 



At present I am not in a position to construct any hypothesis 

 to account for the cleavage mechanism of the ctenophore egg. 

 Further detailed biophysical experimentation on the egg will 

 undoubtedly shed a new light on the problem. As a working- 

 hypothesis this much can be said. Through the action of the 

 centres (centrosomes) sin-face tension is increased along the cleavage 

 plane first at the animal region and then towards the micromere 

 pole^ and thus the ectoplasm is graduall}^ collected. The optical 

 section of the bottom of the cleavage furrow is the '' cleavage head, 

 that is a passive structure. The entoplasm now tends to round 

 up around two centres (geometrical) and the two blastomeres are 

 formed. My experimental study seems to have furnished two 

 important data regarding the above rather vague general 

 interpretation of the cleavage phenomenon. Firstly, the cleavage 

 furrow tends to divide the egg equally, as for instance in the 

 cases where a portion of ooplasm is removed and thus the 

 sjanmetr^^ is disturbed, the new cleavage furrow being bent 

 toward the larger mass of cytoplasm. Secondly, the ectoplasm 

 flows up and down just as Avell without the nuclei and centres, 

 as with them. This change may l)e caused b}^ the unequal 

 increase of surface tension due to the internal division phases. 

 At any rate my results do not indicate that the ectoplasm alone 

 is an active cleaving agent as Ziegler and Rlumbler seem to 

 believe. 



Misald Marine Biological Station 

 Aucr. n. 1910. 



1 As has been pointed out by Zieglïb unilxt->ral cleavaje is doubtless in some way con- 

 nected with the eccentric position of the nuclei and centres. But it should be noted that their 

 being in the ectoplaaui is not ai esi ^ntiil condition of ou»-siied cleavag». In the egg of a 

 good many cœlenterates the nuclei are in the 'n'oplas ii and the clewage is unilateral, c. r., 

 L/H;'r^('s (Conklin), (',i')-ji(>iii<( (Fp[.), Hijilru (Bkaui'^r, T vuneedther). 



