2 l6 



N. YATSU : 



Experiments on the Egg of Strongylocentrotus. 



The cleavage of twelve eggs with extraovates was followed up 

 to the fourth division. Contrary to RAWITZ'S results ('96 p. 78) 

 all of them cleaved more or less in deviation from the normal mode, 

 irrespective of the size of extraovates. As was the case in Arància, 

 the disturbance took place in this form at the place where extraovates 

 were found. Thus in twelve eggs, I examined, four types could be 

 distinguished. In case an extraovate was on the animal region, either 

 one of the eight macromeres was smaller than the others (three cases, 

 one of which is represented in Fig. 6) or it was entirely lacking (three 

 cases, Fig. 7). When an extraovate was situated near the equator, 

 one of the mesomeres was found to be smaller than the other three 

 (two cases). In case an extraovate was attached near the vegetal 

 pole, only three mesomeres and consequently three micromeres were 

 formed (four cases, Fig. 8). 



Conclusions. 



For the study of cleavage of egg-fragments, extraovate experi- 

 ments seem to be in one way more favorable than shaking method, 

 since in the former one can tell at once the exact location and amount 

 of cytoplasm taken off from the egg, while in the latter these matters 

 remain as an inference from the resulting cleavage stages. Of course 

 in both cases, nothing is known about the actual happenings in the egg, 

 i. c, during the flowing-out of the extraovate at the height of turgor 

 of the egg and during the process of breaking-up into pieces while 

 shaken in a test tube. 



It should be especially mentioned that the disturbance of cleavage 

 pattern in the egg with an extraovate is not due to the action of 

 diluted sea-water {cf. DRIESCH '93), because under the same treat- 

 ment, all other eggs (without an extraovate) in the same vessel cleaved 

 normally, and because the disturbance in cleavage, as stated above, 



