CYTOLOGY OF HYDRACTINIA AND PENNARIA. 22/ 



previous to segmentation, the nuclear phenomena in Pennaria ap- 

 pear perfectly normal. 



Sometimes the cleavage is very regular as text-fig. 5 indicates, 

 each cell shows the nucleus in the prophase and apparently so 

 placed as to give off cells in the clockwise directions. The tilt- 

 ing of these nuclei and the perfect regularity is so apparent that 

 this drawing might have been taken from an annelid or mollus- 

 can embryo, but in this same group of cells there was a most 

 irregular embryo so that this regular condition is nothing more 

 than an accident and is the only regular embryo among many 

 sections. 



THE CENTROSPHERE IN THE EGG OF Pennaria. 



It is difficult to decide on any of the old terms to describe the 

 conditions in Pennaria. If the term centrosphere may include 

 sphere and centrosome, the latter being potentially present only, 

 then this may be taken as an acceptable term. 



If there is a centrosome present in these eggs then it is an un- 

 stable body which varies greatly and is recognized with much 

 difficulty. In all cases of the prophase, there remains a small 

 clear area from which the fibers radiate but into which they do 

 not penetrate. Fig. 24, PI. III., shows a few fine granules in 

 this area but their size, number, and staining reaction does not 

 indicate that they are at all constant, nor can one detect that there 

 is any inherent relation between these granules and rays or 

 spindle fibers. That there is probably some substance that plays 

 the part of a centrosome is apparent in Fig. 23, PI. III., where 

 the whole new spindle lies entirely within the old centrosphere. 

 Therefore, it seems as if one might say that certain of these gran- 

 ules have potential centrosome properties and are possibly in the 

 way to become differentiated ; that is to say, that the elaborate 

 centrosomes of mollusca for example indicate a higher degree of 

 differentiation, while in these hydroids a similar result is produced 

 by several granules which cannot be differentiated from the rest 

 of the sphere substance. 



The transformation of this centrosphere keeps pace with the 

 changes in the chromosomes. By the time that the anaphase is 

 reached the centrosphere substance has increased greatly in size, 



