188 ' bulletin: museum of comparative zoology. 



could not be determined. The only evidence of such a body was an 

 aster in the cytoplasm not far from the spermatozoon and near bodies 

 which, though staining rather faintly, appeared to be chromatic. 

 Their condition and arrangement could not be made out with sufficient 

 clearness to determine whether or not thiey were definitive chromo- 

 somes. The surface of the egg was elevated at this point, and it may 

 be that a polar cell was about to have been formed. In the vesicular 

 sperm nucleus represented in Figure 51b, the chromatin is scattered in 

 granules through the reticulum. No asters or radiations of any sort 

 accompanied it. At the surface of the egg there still remained a small 

 cone, but between it and the sperm nucleus there was no "path" in 

 the cytoplasm. Since this egg had a maturation spindle, it seems 

 probable that the spermatozoon may enter before the polar cells are 

 formed. No further stages in fertilization were found, the next more 

 advanced stage observed being that of the first cleavage spindle in 

 metakinesis. 



4. Cleavage. — Segmentation is total but unequal, and it is often 

 irregular in its progress. There is no localization of yolk material to 

 account for this irregularity, which may possibly be due in part to the 

 fact that development takes place in a closed gonophore; but this will 

 not wholly explain the irregularity, for the segmentation of Pcnnaria 

 tiarella as described by Hargitt (:00, lOi'') is strikingly irregular, and 

 yet this egg develops free in the water. Colonies of Tubularia crocea 

 collected from different localities, or from the same place at different 

 seasons, show in some cases (juite regular cleavages; other lots are 

 almost as constantly very irregular. Whether there is any significance 

 in the fact that colonies collected in the fall were more regular in cleav- 

 age than those collected from the same point in the spring, I do not 

 know; but this difference becomes evident upon comparing series of 

 sections of two such lots. It may be stated as a further general con- 

 clusion, supported by all the eggs examined, that cleavage is always 

 accomplished by mitosis. This is especially evident in the early stages, 

 just where Allen (:00) claims to have found indications of the reorgan- 

 ization of a previously fragmented nucleus. It was because of this 

 contention of Allen that especial care was taken to examine closely 

 the early cleavages, and in no instance was there any indication what- 

 ever that a reorganization of fragmented particles was occurring. 

 Furthermore, all cleavages up to the end of segmentation showed no 

 sign of amitotic division, it being possible in almost every case to de- 

 termine the line of descent of each nucleus through preceding mitoses. 



A certain polarity is shown by the egg, though its poles are quite 



