120 George Lejevre 



however, when we are dealing with unusual or abnormal conditions 

 for which no control can be had from living eggs, and here con- 

 clusions can only possess a greater or less degree of probability 

 according to the circumstances of the particular case. 



I The Oocyte and the Maturation Divisions 



The full-grown oocytes, when removed from the tubes, possess 

 a large nucleus placed somewhat eccentrically. The cytoplasm 

 is heavily charged with yolk spheres, which stain a deep black 

 with haematoxylin and are present throughout the entire cell 

 body, except for a peripheral and a perinuclear layer of yolk-free 

 cytoplasm (Fig. 2). The nucleus is filled with a granular reticu- 

 lum which takes the plasma but not the chromatin stains at this 

 time. The large vacuolated nucleolus persists until the breaking 

 down of the germinal vesicle, when it gradually dissolves and dis- 

 appears. The reduced number of chromosomes (12) lie scattered 

 in the nucleus, usually near the membrane, and appear as coiled 

 or twisted granular rods which in some cases are clearly seen to 

 be double or to have the form of rings, or loops, or crosses. As a 

 rule, they stain quite black with haematoxylin and stand out con- 

 spicuously in the lighter, granular reticulum (Fig. 2). During 

 the early prophases of the first maturation mitosis, the chromo- 

 somes undergo a concentration, take up the stain more intensely, 

 and assume the variety of tetrad forms which have been minutely 

 described by Griffin ('99). In this condition they approach the 

 developing asters and finally enter the spindle. 



The formation of the merhbrane and the filling out of the egg to 

 the spherical condition, after treatment with acidulated water, 

 have already been described. It has also been mentioned that 

 in many cases the rtucleus migrates bodily to the animal pole, 

 after the eggs have been returned to normal sea-water (Fig. 3), 

 although this change in position does not always occur. The 

 development of the amphiaster for the first maturation division can 

 only be faintly observed in the living egg, but the process may be 

 followed in great detail from an examination of sections. The 

 earliest material I have in which the asters are unquestionably 

 present was fixed 15 minutes after the eggs were exposed to an 



