122 CHARLES EUGENE JOHNSON 



At a place corresponding precisely to the position in which 

 the third head somite of selachians is situated, Van Wijhe found a 

 solid mass of cells formed from indifferent embryonic mesoderm, 

 which grows forward, becomes associated with the abducent 

 nerve, and gives rise to the Musculus rectus lateralis. This 

 cell-mass he accordingly calls the homologue of the third head 

 somite of selachians. 



Hoffmann ('88), also working on embryos of Lacerta, (L. agihs), 

 found that at a stage in which the optic vesicles are being formed, 

 the first head somites are rather small cavities, one on each side, 

 the walls of which consist of a single layer of cells. These somites 

 are both elongated medially into processes connecting one with 

 the other in the midline. The cavity of the somite does not 

 continue into the process, the walls here being closely apposed. 

 By further development these somites become greatly enlarged, 

 and are then connected by a cross-canal ('Quer-canal') which, 

 at first narrow, soon becomes extraordinarily wide. The end of 

 the notochord lies in close contact with the posterior wall of 

 the canal. Later the canal disappears and out of the walls of 

 the somite are developed those eye muscles which are innervated 

 by the nervus oculomotorius.^ 



Lying above the first gill cleft and just below the ganglion of 

 the N. trigeminus, or exactly in the position where the second 

 head somite of selachians is situated, Hoffmann found a cell-mass 

 conspicuous on account of the peculiar arrangement of its ele- 

 ments. The cells on the periphery are plainly arranged as an 

 epithelium, lie in a single layer, and enclose a rather indistinct 

 cavity, ''so that quite evidently we have to look upon this cell- 

 mass as the homologue of the second head somite." 



A short distance posterior to the second head somite, but 

 somewhat further medially, Hoffmann found in the same develop- 

 mental stage, and on each side, two smaller separate and dis- 

 tinct cell masses, in which also the cells are more or less epithelial 

 in their arrangement, and show traces of a small enclosed cavity. 



^ The muscles supplied by the oculomotor, trochlear, jind abducent nerves 

 respectively, will be frequently referred to as the 'oculomotor,' 'trochlear' and 

 'abducent musclts.' 



