neal: nervous system in squalus acanthias. 



197 



The cavity of the somite is enlarged ventrally opposite that portion of 

 its median surface where a rapid proUferation and migration of cells 

 appears to talie place. I see no reason why the more dorsal and lateral 

 portion of the mesoderm should not be homologized with the myotome 

 portion, and the ventral median region with the sclerotome portion of 

 trunk somites. I am unable to detect any essential diftereuce between 

 the phenomena presented in this section and those presented in sec- 

 tions made in the region of van Wijhe's 5th and 6th somites, to which 

 Eabl grants " Btlrgerrecht" as true somites. The greater dorsal extent 

 of the latter cannot be regarded as an essential difference. Here, as 

 there, we find a well marked and definite region of cell proliferation. 

 As developnjent goes on, the cavity of the 3d somite increases in volume, 

 and at the same time the somite grows forward, chiefly by the elongation 

 of its anterior end, median to the Gasserian ganglion. In confirmation 

 of the statement of Miss Piatt ('91), I find that the first muscle cells 

 are differentiated in the median wall of that portion of the somite which 

 at this stage lies posterior 

 to the Gasserian ganglion. 

 The great extension of the 

 anterior portion seems to 

 retard its histological dif- 

 ferentiation. But in this 

 portion also, wheu muscle 

 cells appear, they are found 

 in the median wall. Eabl 

 ('89, p. 236) says : " Wiih- 

 rend ferner die Muskulatur 

 der Urwirbel ausschliesslich 

 aus der medialen Wand ent- 

 steht, nimmt sie im Vorderkopf zum grossten Theil aus der lateralen uud 

 zum kleineren Theil aus der hinteren Wand der sogenannten Somite den 

 Ursprung." A cross section of the myotome of the 3d somite at a late 

 stage of development appears to me to refute this statement (Figure B). 

 It is clear from an examination of the phenomena presented in such a 



Fig. B. Portion of a cross section throucrh the middle of the myotome of van 

 Wijhe's 3d somite, in a late stage of development (20 mm.). Elontrated muscle 

 cells are already differentiated in the median wall (muscle plate), while the lateral 

 wall (cutis plate) retains its epithelial character. X 50. 



a, cell migrating from the median wall of the myotome into the myocoel; 

 gn. Gas., Gasserian ganglion; la. ct., cutis plate; la. mu., muscle plate; my'cod., 

 mj'ocoel. 



gn. Gas 





 Figure B. 



