124* N. YATSU. 



removed by a pipette. The egg, thus flattened a little, was cut 

 with a fine scalpel. It sometimes proved better, in order to 

 spread the water evenly, to use a little albumen fixative, rubbed 

 on the slide, and thoroughly washed off with a brush. In doing 

 so, the quantity of glycerine left on the slide was infinitesimal, so 

 that it did not affect at all the development of the eggs. My 

 experiments consisted in cutting off a portion of cytoplasm from 

 unsegmented eggs at the four periods already mentioned, in the first 

 two cases fertilizing the fragments, and rearing the resulting em- 

 bryos up to the pilidium. For the sake of uniformity I have 

 confined my work to the nucleated fragments. Cleavage was 

 studied up to the eight-cell stage. Among the pilidia thus ob- 

 tained, I found many abnormal ones, and they were compared 

 with the normal larvae. Every pilidium was drawn with a 

 camera between 48 and 58 hours after fertilization. During 

 drawing they were kept still by sucking out some of the water 

 from under the cover-glass, which was supported by a piece of 

 thread. Thus they could barely move without distortion of their 

 shape. It should here be noted that defective larvse remain 

 always defective ; moreover, the defective parts become more and 

 more prominent as the larvae grow. No size regulation takes 

 place among the pilidia ; the smaller the original egg pieces, the 



smaller the pilidia. 



SERIES A. 



Development of Fragments obtained before the Disappearance of tlic 



Germinal Vesicle. 



A portion of cytoplasm was cut off from the egg immediately 

 after its release and while the germinal vesicle was intact, and 

 the nucleated piece was fertilized. Care was taken not to injure 

 the germinal vesicle, and most of the operations were done upon 

 eggs in which it was eccentrically situated, so as to cut off as 

 much cytoplasm as possible. Since the germinal vesicle, as a 

 rule, lies nearer to the animal pole, it may be inferred that most 

 of my sections were performed in the vegetative hemisphere, 

 although I did not record the exact plane in this series. 



Thirty-five egg-fragments were able to develop up to the 

 pilidium stage, the rest having either died or been rejected on 

 account of polyspermy. The result may be tabulated as follows : 



