EMBRYONIC FISSION IN OYOLOSTOMATOUS POLYZOA. 211 



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 into the aperture. 



Fig. 1 (PI. XXII) 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 in the particular section figured. 



