CYTOLOGY OF HYDRACTINIA AND PKNNAK1A. 2 I/ 



thought to mean the beginning of segmentation, but such a furrow 

 may appear before fertilization takes place. In a number of in- 

 stances the nucleus in this same stage was seen to be partly pro- 

 truding from the substance of the cytoplasm, a condition for 

 which no explanation is offered. The chromatin in the female 

 pro-nucleus takes a faint stain up to the time of the first cleavage. 

 The differentiation of the egg of Hydractinia is very difficult, much 

 more so than in Pennaria, which makes the recognition of these 

 very arduous. In the region of the nucleus in Fig. 5 there are 

 several deeply staining particles which look much like chromatin, 

 but of their nature I am uncertain. That this nucleus is the 

 female pro-nucleus the following reasons indicate: (i) The ab- 

 sence of the nucleolus; (2) its relatively small size; (3) that it has 

 been traced directly into the first cleavage. 



FERTILIZATION IN Hydractinia. 



Thus far no sperms have been found in the eggs before they 

 were deposited but after deposition spermatozoa are seen in con- 

 tact with the eggs. The sperm head becomes transformed into a 

 vesicle soon after it penetrates the cytoplasm. There does not 

 seem to be any definite place where the sperm enters the egg. 

 During the progress of the male pro-nucleus through the cyto- 

 plasm, no aster was seen nor any definite path. The staining 

 reaction of this body is so very faint that it is made out only after 

 careful study with the oil immersion. 



Fig. 6, PI. I., shows the approach of the male pro-nucleus and 

 the change in shape of the female pro-nucleus preparatory to the 

 prophase of the first segmentation. No asters or radiations could 

 be distinguished in connection with either of these pro-nuclei at 

 this stage. These observations further show that the egg nucleus 

 does not " fade from view when the ovum is deposited " but that 

 it can be traced continuously from the egg in the gonophore to 

 the first segmentation stage. 



CLEAVAGE IN Hydractinia. 



The difficulties encountered in differentiating these eggs has 

 made it almost impossible to discover a complete series of the 

 changes in any of the stages as many of the mitotic phenomena 



