l.'J4 University of California Publications. [zoology 



In the rear of the ovieell the continuation of the internode appears 

 in the form of young buds. These would have eventually grown 

 beyond the ovieell and have constituted the remaining zooecia of 

 that internode. Just what stage of development this embryo 

 has attained, it is difficult to say, but judging from others of 

 similar size and appearance it probably consists of three or four 

 blastomeres. 



'Early Cleavage Stages. — It was remarked above that the 

 earliest stages of ovicells are difficult to distinguish. In the 

 sectioned material, no instance has occurred in which a single 

 ovum is contained within an undoubted ovieell. There are 

 many cases of juxtaposition of ovum and group of cells, but 

 as lias been shown the interpretation of this relation is not 

 always possible. It is true that at an early stage an ovieell 

 can be detected by its size, but on sectioning material that 

 could be thus distinguished, cell division has always been found 

 to have occurred. Since in Crista eburnea the ovieell occurs in 

 the proximal portion of the internode, usually in the place 

 of the second or third zocecium, it would seem relatively easy 

 to secure the early stages by preparing in large numbers the 

 young tips of colonies in active reproduction. This method was 

 adopted, but without success in obtaining an undoubted ovieell 

 containing an ovum previous to cleavage. In all of the earliest 

 stages secured, the first cleavage at least had occurred, and there 

 are reasous for supposing that cleavage usually occurs before the 

 ovieell is definitely set off. PL XIII, Figs. 19 and 20. are con- 

 secutive sections of one of the three youngest ovicells obtained. 

 The embryo consists of two blastomei*es, one being represented 

 in each figure {bl.). These latter are large ovum-like bodies 

 imbedded in cells and lying distal to a mass of elongated cells 

 which represent the polypide bud of an ordinary zocecium (/»/. bd: ) , 

 and which seem to be arranging themselves around the embryo 

 to inclose it. The cells of the embryo possess a large vesicular 

 nucleus, and in size and appearance bear so close a resemblance 

 to ova, that the question arises whether they may not be such. 

 The strongest evidence that they are the result of cleavage is 

 found in the relative size of nucleus and cell. Measurements of 

 a large number of ova show that the ratio of the size of the 



