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



postotic somites, give rise to the six muscles of the eye,* which are distinctly 

 differentiated already at the close of the ninth day or at the commencement 

 of the 10th day. The preotic section of the head is consequently totally 

 destitute of the segmental muscles derived from the somites. This deficiency 

 is, as pointed out by Kupffer, made good by a few myotomes behind the 

 auditory vesicle, the dorsal parts of which are bent and shifted forwards into 

 the head. 



The mesectoderm is, therefore, the product of that part of the mesodermic 

 somites which corresponds to the cutis layer. The dorsal division, which is 

 distinguished by Koltzoff from the ventral division, is derived from the three 

 preotic somites and confined to the preotic section of the head ; it never 

 assumes an epithelial structure, but -makes a simple network with the nerve- 

 cell strings. 



On the contrary, the ventral division is a continuous layer of typical 

 epithelium, extending from the anterior margin of the lateral plates to the 

 posterior boundary of the branchial region and as high as the lateral plates. 

 This division of the mesectoderm is cut into nine vertical epithelial bands, 

 when the lateral plates are divided into nine visceral arches. 



That part of the mesectoderm which assumes the epithelial character in 

 the snout may be regarded as the anterior continuation of the ventral division. 

 If this assumption is correct, all the three preotic somites, just like the 

 following somite in the postotic branchial region, contribute elements to the 

 ventral division of the mesectoderm. 



2. Fate of the Mesectoderm. 



While the nerve-cell strings in the pre- and postotic region are transformed 

 into the nerves and ganglia, the mesectodermic elements of the dorsal division 

 are converted mainly into the connective tissues standing in relation to the 

 nerves and ganglia ; a small portion of them, which lies in contact with the 

 medullary walls, gives rise to the most anterior section of the cranial skeleton, 

 i.e. the trabecular 



The nine mesectodermic bands, into which the continuous layer of the 

 ventral division is divided by the visceral invagination of the pharynx-walls, 

 undergo the following differentiation. On frontal sections through a just- 

 hatched larva, the first stage of the differentiation is very obvious. At the 

 level of the visceral pouch, the entodermal wall of the pharynx is in close 

 contact with the ectoderm, while at the level of the visceral arch, between 

 the two layers, are contained the vascular cells, the mesodermic arch and the 



* Detailed accounts on the development of the ophthalmic muscles I have given in my 

 above-mentioned paper (146). 



