516 OR MASTERMAN ON THE FURTHER ANATOMY AND 



A median sagittal section through this bud (fig. 37) shows but little advance in 

 differentiation upon the last stage. The endodermic pouch has become larger and 

 reaches further backwards than in earlier stages. At the dorsal lip of the mouth is seen 

 a slight depression, which is the first trace of the sub-neural gland (sn.gl, Harmer's 

 notochord). In such a case as this, where the whole endoderm is derived from the 

 ectoderm, it is impossible to locate the origin of the sub-neural gland in any special 

 embryonic layer, but it may be noted that its first appearance is much further outside 

 the pharynx and away from the sub-neural sinus than later. 



Subsequent growth results in a clearer definition of the three areas already referred 

 to, and in the appearance of two dorsal rounded knobs immediately behind the buccal 

 shield (figs. 21 and 22). Sections indicate some important changes in the internal 

 structure. In a horizontal longitudinal section (fig. 38) it is seen that the ectoderm 

 has become drawn in behind the buccal shield, and where the pedicle meets the body. 

 The rapidly enlarging endodermic sac touches it at these two parts forming two 

 circular lines of contact. In this way the single ccelomic space is nipped off into a 

 single pre-oral cavity (protoccele), a pair of collar-cavities (mesocceles) still separated by 

 the persistent median mesentery and a pair of trunk-cavities (metacceles) which are still 

 continuous through the pedicle with the ccelome of the parent. 



A stage very like this was figured by Dr Harmer (1) and led him to compare the 

 bud of Cephalodiscus at this stage with the young Balanoglossus. 



The cavities of the two knobs, which are the first pair of plumes, are in direct con- 

 tinuity with the two collar-cavities. Fig. 39 is a median sagittal section through the 

 bud shown at fig. 22. We may here note that the endodermic sac has increased in 

 size and the sub-neural gland has moved backwards towards the sub-neural sinus. The 

 dorsal posterior lip of the hind-gut has commenced to move forwards along the mid-dorsal 

 line. Between this stage and that shown in fig. 37, the hind apex of the endodermic 

 sac touches the dorsal ectodermic wall, and then from this point the intestine grows 

 forwards as seen in fig. 39, till it eventually opens to the exterior, a little way behind 

 the sub-neural sinus. 



In this manner the dorsal reduplication of the gut is brought about, and it seems 

 reasonable to suppose that the spot where the endodermic sac first touches the ectoderm 

 represents the primitive posterior position of the anus, the later forward growth of the 

 intestine indicating a phyletic forward movement of the anus (cf. Segmental duct of 

 Vertebrata). The endodermic sac never comes in contact with the wall of the bud 

 between the mouth and the pedicle, so that the pedicle could not be regarded as a 

 dorsal process. 



The next important external change is the separation of the collar-area from that 

 of the trunk (fig. 23), caused by a bulging outwards of the former. At this stage the 

 trunk is also more clearly separated from the pedicle and their line of junction is now 

 on the under-side. The two plumes have elongated and their distal extremities have 

 swollen out to form the compound eyes. 





