BUDDING PROCESSES OF CEPHALODISCUS DODECALOPHUS. 515 



This protuberance becomes constricted at the base and then appears as a small 

 knob (fig. 19) which presents no external differentiation. 



The internal structure of this bud can be understood from the sections shown in 

 figs. 29-32, selected from a series cut transversely to the long axis. 



At the distal extremity (fig. 29) a thick layer of ectoderm, containing a mass of 

 nuclei and ill-defined cell-walls, is lined on the inside by a thin layer of mesoderm, in 

 which are scattered a few nuclei, Protoplasmic threads of mesoderm interlace across 

 the central ccelomic lumen. 



In fig. 30 the ccelome is shown to be divided in the middle by a mesentery, running 

 as is seen later, dorso-ventrally. Within this mesentery (fig. 31) is formed a blood- 

 sinus (m.s.), which may be followed through the pedicle (fig. 32) into the ventral blood- 

 vessel and mesentery of the pedicle (see fig. 18). At the level of fig. 30, the median 

 blood-sinus has moved dorsalwards and merged into a large dorsal sinus (sn.s.) under- 

 lying the ectoderm which proves to be the rudiment of the sub-neural sinus. 



A sagittal longitudinal section through a similar bud is seen in fig. 36. If 

 it had been exactly median, it would have shown the median sinus (m.s.) continuous 

 with the sub-neural sinus. 



Further development proceeds by a rapid growth of the apical portion of the bud 

 resulting in the formation of the pre-oral lobe (protomere or buccal shield), one portion 

 of which protrudes dorsally immediately in front of the sub-neural sinus, and grows 

 out ventrally to such an extent that the bud assumes the appearance of being bent 

 upon itself (fig. 19, left-hand bud). 



A transverse section through the pre-oral lobe (fig. 33) shows a thickened ventral 

 wall and a coelomic cavity in continuity with that of the rest of the bud. 



A section at the base of the pre-oral lobe (fig. 34) shows the sub-neural sinus and 

 the median mesentery as in fig. 30, but, in addition, the ectoderm is invaginating, 

 forming a diverticulum (g.) pushing into the median sinus and gradually downwards 

 between the two walls of the mesentery (fig. 35). Below the level of fig. 35 a cross- 

 section presents the same character as fig. 32. 



The commencing invagination of ectoderm shown in figure 34 gives rise, as may be 

 easily demonstrated in later stages, to the whole of the alimentary canal of the adult, 

 the orifice of invagination persisting as the mouth, and the anus being a new formation. 

 The endodermal layer of the parent plays no part whatever in the formation of the 

 bud, the whole future endoderm being derived from the ectoderm of the bud. 



It is important to notice that the first invagination takes place opposite the sub- 

 neural sinus and immediately behind the buccal shield, which is therefore pre-oral in 

 position from the outset. 



In the next stage the buccal shield has grown out laterally as well as in the dorsal 

 and ventral line (fig. 20), so that the body now appears to consist of three parts, the 

 buccal shield, the body in which is contained the future trunk and collar and the long 

 cylindrical pedicle, which has commenced to elongate. 



VOL. XXXIX. PART III. (NO. 17). 4 H 



