194 The Development of the E3^e Muscles in Acanthias 



away, and a considerable space is left beneath it. Into this space a 

 proliferation of mesenchyme cells takes place from the anterior7median 

 wall of the somite. 



The walls of the somite at this stage are thin and single-layered^ 

 except in the median side, where the cells are higher and a tendency 

 towards the formation of two layers is evident. Later, the median 

 wall becomes thicker, and the area of mesenchyme proliferation more 

 definite. The cells derived from this outgrowth are spreading out 

 around the walls of the somite. This outoTowth seems to me to be 



Fig. 6. — Reconstruction of optic vesicle and derivatives of the premandibular 

 somite of an Acauthias embryo, 2(5 mm. total length. Right half, medial view. 



comparable to a sclerotome of a trunk sonute, both in position and in 

 histological appearance. 



At a 10 mm. stage the lateral, as well as the posterior median walls, 

 have become very thin. This thinning out at posterior median wall 

 continues, and soon the epithelial character of the bounding tissue is 

 lost. This break, together with a constriction which gradually takes 

 place, divides the somite at this point ultimately into a dorsal and a 



