PLASMA-STRUCTUKE IN EGG OF HYDRACTINIA 225 



for Clava by Harm ('02) who finds the first polar spindle very 

 small and formed within the germinal vesicle from the achromatic 

 portion, and the same condition in Cordylophora, as described 

 by Morningstein ('01). 



3. Fertilization 



The egg is fertilized as soon as it is shed, the spermatozoon 

 entering at any point on the surface. A f^ilization-membrane is 

 formed (fig. 42). The method of union of the two germ-nuclei 

 depends on the point at which the spermatozoon enters. If, as 

 often happens, it enters near the female germ-nucleus (fig. 47) the 

 sperm head may en,ter the egg-nucleus bodily, without expanding 

 (fig. 49). Here the chromatin of the egg-nucleus is already col- 

 lected in masses to form the chromosomes of the first cleavage- 

 spindle. An aster accompanies the sperm head in this case as in 

 others, no spindle as yet having formed, however. If the sperm 

 enter at some distance from the female germ-nucleus, it enlarges 

 as 'Usual before union and a spindle develops in connection with 

 it. The 'degree of enlargement varies, as shown in figures 48 and 

 50. Complete fusion of the two nuclei may take place before the 

 breaking up into chromosomes but this is not essential. 



The entrance of the sperm head directly into the egg nucleus 

 without expansion is 'described in some other Hydroids (Wulfert, 

 '02, Gonothyrea; Harm, '02, Clava squamata ; I have also observed 

 it in Pennaria) . Also as the figures show, I do not find the sperm 

 head in Hydractinia forming a group of vesicles as figured by 

 Smallwood ('09) in Hydractinia and Smallwood ('09) and Hargitt 

 ('09) for Pennaria. -It expands directly into a single vesicle. 



4- Fertilization -membrane and mitochondria 



The hydroid egg is usually spoken of as naked (Wilson '00, 

 Hargitt '04). Smallwood ('09), however, has described a mem- 

 brane for Pennaria, formed at fertilization. I have found this 

 membrane easy to demonstrate in fresh material if intra vitam 

 staining methods described by Kite ('12) are used. In Hydrac- 



