518 DR MASTERMAN ON THE FURTHER ANATOMY AND 



Returning to figure 43, we see here only the collar-cavities (c.c.) and the dorsal 

 sinus, whilst the base of the sub-neural gland is shown to open to the exterior in the 

 mouth region. On the right-hand side the oral groove is recognisable. In fig. 44 both 

 oral grooves arc seen, and in fig. 45 the mouth is freely open ventrally. The collar- 

 cavities are divided by a pair of trunk-cavities which lie on either side of the median 

 dorsal mesentery. On the right hand appears an oviduct (g.d.) arising as an invagina- 

 tion of the ectoderm. The cross-section of the pharynx here shows a pair of lateral 

 grooves which are the first trace of the pleurochords. 



In fig. 46 we can note the paired ectodermal oviducts (g.d.) and below them the 

 trunk-cavities. Laterally are the collar-cavities, and ventrally are the two collar-pores 

 (c.p.) arising in a precisely similar manner to the proboscis-pores and the "oviducts" 

 as pair ectodermal tubes. 



In fig. 47 the stomach (st.) presses against the dorsal w r all, and the trunk-cavities 

 and ventral mesentery complete the section. In the mid-ventral line can be discerned a 

 commencing nervous differentiation. 



Fig. 48 is cut at the commencement of the pedicle, and shows the intestine in 

 contact with the dorsal wall. 



In this feature the bud is at an earlier stage than that which is shown in fig. 

 39, where the intestine has commenced its forward migration. 



The mesentery is now incomplete, and the ventral sinus runs down to meet the 

 ventral sinus of the parental pedicle. The mesoderm is partially differentiated in the 

 ventral area into muscular tissue, which will later become the longitudinal muscles. In 

 the ectoderm outside this the ventral nerve-tracts are clearly distinguishable. They 

 are continuous with those of the adult. 



On either side of the pedicle is a lateral groove (l.g.) which is of constant 

 occurrence, and gives the pedicle at this stage a characteristic cross-section. The basal- 

 fixing extremity of Phoronis presents a similar pair of lateral grooves most clearly 

 defined immediately after metamorphosis. In the buds of Cephalodiscus they disappear 

 early (see fig. 59), and their meaning is doubtful. 



From this series and the above description can be obtained an idea of the structure 

 of a typical bud of Cephalodiscus, with one pair of plumes. 



The simple archimeric segmentation into five archicceles, the simple and non- 

 reduplicated gut, and the primitive blood-system of sinuses should be emphasised, 

 whilst the identity in structure and origin at this stage of the proboscis-pores, the 

 collar-pores and the " oviducts," is very striking. The proboscis-pores alone appear to 

 open into the ccelome. 



Fig. 24 is a dorsal view of a bud intermediate in external character between the bud 

 with one pair of plumes, just described, and that with two pairs, shown in figs. 25 

 and 26. 



The second pair of plumes are seen to arise near the base of the first, as simple 

 horn-like protuberances, whilst the first pair have developed a number of pinna) from 



