62 EMBEYOLOGY OF THE LOWER VEETEBEATES ci-i. 



groove of the inner surface of the wall of the archenteron so that 

 where the junction exists the archenteric cavity may be said to pro- 

 ject slightly into it. The cells at this point develop pigment in their 

 protoplasm (Fig. 37, B); they frequently show mitotic figures and 

 appear to be actively proliferating, cells being added at this point to 

 the mesoderm. 



The peculiarities which ■ have just been described, and which 

 occur in various amphibians, have important bearings in two different 

 directions. In the first place they form an important part of the 

 basis for 0. Hertwig's hypothesis of mesoderm formation in the 

 Vertebrata, the junctions, which have just been described, between 

 endoderm and mesoderm being interpreted by him as representing 

 the original stalks of the mesoderm segments as they occur in Am- 

 phioxus. As already indicated there do not appear to the writer to 



B 



n.p. 



2d. 



N. 



Fig. 37. — Transverse sections through embryos of (A) Triton and (B) Rana temporaria 

 showiDg continuity of endoderm and mesoderm on each side of the notochord. 

 (After 0. Hertwig, 1882 and 1883.) 



end, endoderm ; m.p, med ullary plate ; mes, mesoderm ; N, notochordal rudiment. 



be sufficient reasons for regarding these connexions as primitive 

 rather than those more ventrally situated. The balance of probability 

 appears rather to favour the view that of the two connexions it 

 is the ventral one which is the persistent original one, and that 

 it is the dorsal which is to be interpreted as due to secondary 

 fusion. 



The second bearing is at least equally important. It rests on 

 the occurrence of active cell-proliferation on each side of the noto- 

 chordal rudiment. For in some of the meroblastic vertebrates 

 (Amnio ta) — correlated with the more and more complete segregation 

 of yolk from protoplasm — this zone of proliferation becomes ap- 

 parently the main source of the mesoderm. 



Elasmobranchii. — In the Elasmobrancb, while there are still 

 traces of formation of mesoderm by a process of delamination from 

 the main mass of endoderm or yolk (Fig. 38, A), a more con- 

 spicuous mode of formation is provided by active proliferation of 



