60 ZOOLOGY SECT. 



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 Balanoglossus 

 (p. 8) ; — 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 backward extensions, 

 which may be compared to the collar-cavities ; and a posterior pair correspond- 

 ing with the trunk-ccelome of the Enteropneust — the last becoming divided up 

 to form the ccelomic sacs. 



New ccelomic 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. 753, 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. 24) : 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 ccelome of the head (A), the left to 

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

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

 afterwards formed. Hatschek' s nci^hridimn (Fig. 754, x) is a narrow 

 ciliated tube which opens into the anterior part of the pharynx, 

 and runs forwards to terminate blindly in the roof of the 

 oral hood. It appears to be developed from the narrow 

 neck that connects the left ccelomic pouch of the first pair 

 with the archenteron, and disappears completely in the adult 

 except in Amphioxides, in which it is said to contain solenocytes. 



On the floor of the archenteron in the neighbourhood of the 

 mouth a depression appears giving rise to a structure known as 

 the chib-shaped gland (k) which may be a modified gill-cleft. 

 Posteriorly the neurenteric canal closes and the anus appears. 



We left the mesoderm in the form of separate paired ccelomic 

 sacs, arranged metamerically in the dorsal region of the embryo. 

 The sacs increase in size, and extend both upwards and downwards, 

 each presenting a somatic layer (Fig. 751, D, mP) in contact with 

 the extei'nal ectoderm, and a splanchnic lager {nikP) in contact 



