58 ZOOLOGY SECT 



neuron, never exhibits any trace of segmentation. At its first 

 formation it stops short of the anterior end of the archenteron : its 

 final extension to the end of the snout is a subsequent process. 



The significance of these early stages in the development of Amphioxus has 

 been variously regarded by different embryologists, and it is impossible to give 

 here more than a statement of the ascertained facts, leaving the question of 

 their interpretation, which can only be profitably discussed on the broadest 

 basis, as a matter for more advanced study. Since, however, the gastrulation 

 and mesoderm-formation in Amphioxus are, almost universally, made the 

 starting-point in the process of interpreting the phenomena of early develop- 

 ment in all Vertebrates, it may be desirable at the present stage to state that 

 though in the foregoing account the cells wlu'ch are invaginated are referred to 

 throughout simply as endoderm, this view of their nature is not the only one 

 that may reasonably be held. If the inner layer of the gastrula be composed 

 of endoderm and of endoderm only, certain consequences necessarily follow : 

 the notochord must be purely endodermal in origin, and so must the whole of 

 the mesoderm. This is the terminology which has been followed in the 

 preceding pages. But it may be held that the process of invagination is not 

 so simple, and that the inner layer of the resulting gastrula is made up of two 

 distinct parts, a dorsal part, which is ectodermal, and a ventral part, which 

 consists of endoderm. On this view the notochord and the mesoderm, derived 

 from the dorsal part of the invaginated layer, would both be of ectodermal 

 origin, and only the enteric epithelium would be endodermal. These points 

 will be referred to again at a later stage. 



A further point about which there may be room for differences of opinion is 

 the detailed development of the ccelome. According to one view of the facts 

 the ccelome of Amphioxus may at one stage be compared to that of Balano- 

 glossus (p. 3) : with an unpaired anterior part, destined to form the head- 

 ccelome and representing the proboscis- cavity of Balanoglossus ; a middle pair 

 of pouches which form the first pair of somites, and a pair of canal-like back- 

 ward extensions, which may be compared to the collar-cavities ; and a posterior 

 pair corresponding with the trunk- ccelome of the Enteropneust the last 

 becoming divided up to form the ccelomic sacs. 



New coelomic pouches are formed in regular order from before 

 backwards, the embryo at the same time elongating and becoming 

 laterally compressed and pointed fore and aft. At the anterior end 

 the mouth (Fig. 762, m) appears on the left side of the body as a 

 small aperture, which soon increases greatly in size. On the ventral 

 surface another small aperture, the first gill-slit (ks), makes its 

 appearance, and soon shifts over to the right side : it forms a 

 direct communication between the pharynx and the exterior, like 

 the stigmata of Appendicularia (p. 23) : there is at present no trace 

 of the atrium. 



The anterior end of the archenteron has meanwhile grown out 

 into a pair of pouches, which become shut off as closed sacs : of 

 these the right gives rise to the coslome of the head (h), the left to 

 a depression called the pre-oral pit (w), which opens on the exterior, 

 and from which the groove of Hatschek and the wheel-organ are 

 afterwards formed. The pre-oral pit also gives rise to Hatschek 's 

 nephridium (Fig. 763, x), a narrow ciliated tube which opens into 

 the anterior part of the pharynx, and runs forwards to terminate 



