Budding in Perophora. 137 



The first appearance, of a bud is announced by the thick- 

 ening at a given point of the ectodermal wall of the stolon, 

 which becomes slightly raised up into a hemispherical pro- 

 tuberance. The ectodermal cells, which are elsewhere flat 

 and attenuated, are here nearly culxjidal. 



By making a cross-section through the stolon at such a 

 place it is found that the walls of the stolonic partition are 

 no longer closely pressed together, but are wide apart, and 

 greatly thickened on the side next to the bulged-out ecto- 

 derm. 



The thickened portion of tlie partition soon grows out into 

 the ectodermal evaginatioii, and becomes closed off as a vesicle 

 by the corning together of the walls of the partition inside the 

 stolon proper. 



This evaginated thickened part of the partition forms the 

 " endodermal vesicle " of the bud-rudiment, and its cavity is 

 to be regarded as being in communication for some time with 

 the potential cavity of the partition, as the connexion with the 

 latter is not lost until a much later stage. 



When the protrusion of the bud-rudiment beyond the 

 surface of the stolon has proceeded to a certain extent the 

 ectodermal connexion becomes somewhat constricted, and the 

 rudiment, which has now a spherical form, stands out on the 

 stolon as a round knob. It does not long retain this shape, 

 but soon begins to elongate in the direction of the free tip of 

 the stolon. It is now ellipsoidal in form, with its long axis 

 parallel to the long axis of the stolon, and one end lies free 

 over the surface of the stolon beyond that portion which is 

 directly connected with the latter. It is this free end which 

 is later to be distinguished as the anterior part of the bud, 

 while by the process of elongation anteriorly the connexion 

 with the stolon comes to be left behind in the posterior region. 

 The side next to the stolonic wall becomes the ventral surface 

 of the bud and that op{)osite it the dorsal. 



V>y this time the rudiments of several structures begin to 

 appear. They do not all arise simultaneously, however, and 

 although the pericardium is the earliest to be laid down, it 

 will be necessary to describe the formation of the peribranchial 

 sacs first, in order to render intelligible certain relations 

 between these and other structures. 



The Ferihranchial Cavity. 



The observations on the development of this structure in 

 Perophora viridis, although agreeing with those of BIjort* on 



* Mitth. Zool. Stat. Neapel, 10 Bd., 1890. 

 Ann. & Mag. A. Hist. Ser. G. Vol. xviii. 10 



