BUDDING PROCESSES OF CEPHALODISCUS DODEGALOPHUS. 519 



the apex downwards. The further development of the tentacles will be referred to 

 later, though it may be here stated that the stage to which, development of the plumes 

 has reached is only a rough criterion of the amount of internal differentiation. Thus 

 the sections of fig. 24 to be described are of as advanced a stage as those cut through 

 fio-s. 25 and 26, and seem to present, as far as is possible, the stage indicated externally 

 by the presence of two pairs of plumes. 



Figs. 49 to 59 represent this stage in sections. 



In fig. 49 there is shown a considerable advance over fig. 40. The nervous layer is 

 well developed, both dorsally and ventrally, in the buccal shield and in the dorsal 

 ectoderm of the plumes. The sub-neural sinus has thickened mesodermic walls, prob- 

 ably contractile. 



Fig. 50 shows the sub-neural gland in cross-section, now of much less proportionate 

 size than in fig. 41. 



In fig. 51 the post-oral lamella and collar-cavities are evident, and a cross-section of 

 the pharynx has a very characteristic outline. The median dorsal and median ventral 

 grooves are continuous with the aperture of the sub-neural gland. The paired dorso- 

 lateral grooves are the commencing pleurochords, and the ventro-lateral grooves are the 

 paired oral grooves. These latter run in fig. 52 to the exterior. 



The rectum, bounded by the trunk-cavities laterally and the oviducts (g.d.) running 

 in lateral mesenteries, are also seen in this figure. 



In figs. 53 and 54 the ovaries (ov.) developing from the ccelomic wall, and the 

 post-oral lamella should be noted, whilst the latter figure shows, on the left side of 

 the pharynx a pharyngeal pouch, not yet opening to the exterior (p.c). Its formation 

 from an evagination of the pharyngeal wall is in accordance with its structure in the adult. 



The pharynx (ph.) stomach (st.) and rectum {in.) are all cut in fig. 55 behind 

 the gut (fig. 58), the dorsal and ventral sinuses merge into one, to again separate 

 in the pedicle, in which the mesentery is interrupted (fig. 59). The longitudinal 

 muscles, traces of which may be found at the extreme front-end of the pharynx 

 (fig. 53), become very prominent, and are of an advanced histological structure 

 posteriorly (fig. 57). They extend round the whole ccelome in the pedicle (fig. 59). 

 The single ventral nerve-tract in this region spreads out further forwards (fig. 56) 

 and can be traced upwards as a pair of lateral cords (fig. 55) and (fig. 54) into the 

 post-oral ring. 



The external appearance of the four-plumed stage is indicated in figs. 25 and 26. 

 The buccal shield is of enormous size, greater in proportion than in the adult, and 

 the median curved line of pigment is distinct. In front view (fig. 26) it is saddle- 

 shaped, and in side view (fig. 25) it shows a thickened rim. 



This figure shows that the pedicle, at first terminal, and in the direct axis of the 

 body, becomes apparently ventral by the dorsal protrusion of the trunk and its 

 contained structures. Thus the pedicle becomes shifted till it appears, in figs. 27 and 

 28, and still more in the adult, to be an appendage of the ventral body- wall. 



