14 ART. 4. — N. YATSU : 



V. 2-CELL STAGE UP TO BLASTÜLA. 



On the fading away of the membrane of the cleavage 

 nucleus the mitosis begins, whose figure lies in a somewhat clear 

 space stained faint blue by iron-hsematoxylin. The chromo- 

 somes are thread-like during the pro- and earlier ana-phases (PI. 

 II., Figs. 23, 24.), but in the later anaphases they are found to 

 shorten into small rods and to occupy either end of the spindle, 

 at this time centrospheres and centrosomes becoming invisible 

 (PI. II., Fig. 25). 



Next a constriction goes on to some extent around the egg, 

 but it must not be taken as a part of the cleavage phenomena. 

 In the course of about half an hour the ovum regains its original 

 spherical form. I afterward found that the same thing occurs 

 also among Mollusks. It is the second constriction which now 

 appears that divides the egg into two blastomeres. At the be- 

 ginning of the first cleavage the yolk layer seems to be concentrated 

 about the nucleus of each blastomere, the vacuolar layer entering 

 between the two blastomeres. Meanwhile the constriction becomes 

 deeper and deeper until the egg is divided into two, the cell 

 membrane being formed between them (PI. I., Fig. 2 ; PI. II., 

 Fig. 26.). 



The nucleus is vesicular with a thick and strong looking 

 membrane : chromatin is very small in quantity, diffusely dis- 

 tributed on the nuclear reticulum. These features of the nuclei 

 last for a long time until histological differentiations take place. 



As the egg is quite spherical, the yolk is distributed equally, 

 and moreover as the polar bodies are soon absorbed, it becomes 

 impossible to orient the first cleavage plane with reference to 

 the future body axes. 



