EMBEYONIO FISSION IN CTOLOSTOMATOUS POLYZOA.. 13 



branch; of a second zooecium, which is very nearly mature; 

 of the ovicell as the third member of the internode ; and of 

 the real growing-point, which contains a young polypide-bud ; 

 although the corresponding zooecium is not yet separated off 

 from the growing-point by a septum. 



The ovicell contains a structure which is the exact equivalent 

 of an ordinary polypide-bud. This consists of (1) a thick 

 (proximal) mass of cells, which in a zooecium would give rise 

 to alimentary canal and tentacles; (2) a thin-walled portion, 

 next to the above, and corresponding to the tentacle-sheath; and 

 (3) a distal portion, indicated by two parallel lines in the sketch, 

 and which is really an invagination of the distal body-wall of 

 the ovicell. This is formed in a precisely similar manner in 

 any young zooecium, where it develops iuto the aperture. 



Fig. 1 (PI. I) is a nearly median longitudinal section 

 of an ovicell at nearly the same stage as fig. 15. The body- 

 cavity is, as in ordinary zooecia, largely filled up by funicular 

 tissue, but contains an obvious polypide-bud, the distal portion 

 of which can be clearly distinguished as a tentacle-sheath, 

 similar in all respects to the same structure in an ordinary 

 polypide-bud. The one fact, indeed, which enables this member 

 of the colony to be distinguished as an ovicell is the presence 

 of a relatively large cell, which is closely applied to one wall 

 of the polypide-bud. The latter shows some tendency to give 

 off cells which are growing round the large cell. This has a 

 diameter of about '0176 mm., and it has a large clear nucleus 

 with one or two nucleoli. Its structure, in fact, reminds one 

 irresistibly of that of an egg ; and I believe this cell to be the 

 source from which all the larvae produced in the ovicell are 

 developed. 



"Eggs" of this kind are found in various positions in some 

 of the growing.points. Thus in the particular individual in 

 question there is a second, smaller egg^ in the same ovicell ; 

 and in the next zooecium there are two eggsi, one of which is 

 at the apex of the polypide-bud. The fact that these eggs are 

 commonly found in the growing-points leads me to suppose 

 ' Not visible ia the particular section figured. 



