464 Mr. S. Hatta. Mesodermic Origin and the Fate of the 



mentioned edge of the scleromyotome, and wedging itself in between the 

 meso- and ectoderm. 



The manner in which the mesectoderm develops reminds us of the folding 



Text-fig. 2. Text-fig. 3. 



Text-fig. 2. — Diagrammatic representation of a transverse section through a post- 

 branchial visceral arch, showing down-growth of the cutis-layer of scleromyotome 

 to give rise to mesectoderm (left) and the established mesectoderm (right). The 

 mesectoderm and its rudiments are shaded, ct., cutis layer ; sch., subchordal cells ; 

 thy.g., thyroid groove. For other letters, see the explanation of the previous 

 diagram. 



Text-fig. 3. — Diagrammatic representation of a frontal section showing the position of 

 the mesectoderm in relation to other layers. The posterior visceral pouches are in 

 formation, and the lateral plates are being cut into the mesodermic visceral arches. 

 The mesectoderm is shaded, b., brain ; bcl., branchiocoele ; ep., epidermis ; I., placode 

 of lens ; ma., mesodermal visceral arch ; me., mesectoderm ; oc, optic cup ; pt., 

 pronephric tubule ; v.va., rudiment of vascular arch ; sc., splanchnoccele ; spp., 

 splanchnopleure ; sp., somatopleure ; St., stomodseum ; vp., visceral pouch. 



of the corresponding edge of the scleromyotomes in the postbranchial region, 

 by means of which the scleromyotomes grow downwards so as to provide the 

 ventral somatic walls of the body with the muscular and the cutis layers. 

 The downward growth of the scleromyotomes in both cases is, I venture to 



