262 MARGARET MORRIS HOSKINS. 



and an abnormal number of chromosomes is formed. It is not 

 possible to determine whether the mass of chromatic material 

 rep esented in one of these plates of chromosomes is exactly 

 equal to that of a normal cleavage plate, but on theoretical 

 grounds it was assumed that such is the case. 



(b) Suppression of Maturation in Fertilized Eggs. As has been 

 said the fertilized eggs in which maturation is to be suppressed 

 are heated within five minutes after their insemination. At this 

 time, the polar spindle has moved to the periphery of the egg, 

 but the formation of the polar body itself has not begun. The 

 sperm head is within the egg, but has not been transformed into 

 a vesicular pronucleus. The first stage that is of interest in 

 eggs preserved from these experiments is the anaphase repre- 

 sented in Fig. i. In this it will be seen that a division of the 

 eighteen bivalent chromosomes has taken place, and that the 

 daughter chromosomes are moving somewhat irregularly to the 

 poles of the spindle, while the sperm head is beginning to enlarge 

 to form a vesicle. 



A later stage of this nuclear division is shown in Fig. 2. Here 

 the chromosomes have begun to form vesicles preparatory to 

 passing into the resting stage. It will be seen from Fig. I that 

 although the division of the chromosomes is complete, the migra- 

 tion of the daughter chromosomes to the poles of the spindle is 

 somewhat irregular, and it is owing to this fact that we do not 

 find two perfectly definite groups of vesicles in such a stage as 

 Fig. 2 represents. In this figure the male pronucleus is also to 

 be seen ; it is well advanced in its development, and apparently 

 has not been injured by the heat. 



Further development of the resting nuclei is seen in Fig. 3. 

 Here two daughter nuclei are found, with a third small vesicle 

 which would undoubtedly have fused ultimately with them, as 

 there is no evidence that any such vesicles are ever left out. In 

 this stage the male pronucleus had developed still farther, and it is 

 probable that the aster represented in the figure is of male origin. 



Fig. 4 represents the final stage of the suppression of the polar 

 body. In this, the two daughter nuclei formed from the chromo- 

 somes of the polar spindle have fused completely. The male 

 pronucleus has reached a large size and lies closely apposed 



