CYTOLOGY OF HYDRACTINIA AND PENNARIA. 221 



always present in the egg previous to the beginning of seg- 

 mentation. 



Sections of medusae after they have become free from the 

 colony and before the eggs are discharged may show the pres- 

 ence of polar bodies. Several different slides show conditions 

 such as are indicated in Fig. 10, PI. II. The first polar body is 

 being shoved to one side by the formation of the second. The 

 chromatin is in the form of three vesicles in the first polar body. 

 Even while the egg is in the medusa, the polar body may be 

 pushed some distance from the spot where it emerged which may 

 be due to the contractions of the bell of the medusa. The sec- 

 ond polar body has ten vesicles in two groups ; while thirteen are 

 made out deeper in the egg. To the right there is one isolated 

 vesicle. This one and several of the thirteen contain granules of 

 chromatin. This interpretation of the vesicles grows out of my 



FIG. 5. A regular segmentation stage, all cells in prophase. 



study of the changes through which the chromatin passes during 

 cleavage. The several vesicles deep in the egg unite into a sin- 

 gle definite nucleus like the one in Fig. 11, PI. II. That these 

 polar bodies are formed by the mitotic process, there is but little 

 doubt because of similar conditions in Hydractinia where there 

 can be no doubt as to the beginning of the process ; and because 

 of the state of the chromatin, which in this condition of several 

 vesicles, is entirely unlike the amitotic process of division. 



In the same medusa from which Fig. 10 was taken there were 



