188 The Development of the Eye Muscles in Acanthias 



large mass of cells rather loosely packed, and occupying the space be- 

 tween eye-ball and epidermis. The somite is pressed closely against the. 

 anterior walls of both the 1st and the overlapping 2nd somite. 



As the embryo grows older the somite assumes a much more elongate 

 form (Fig. 1). The lumen increases in size and extends far into the 

 ventral process, while the walls become considerably thinner and but- 

 one cell in thickness; still, they never become so thin as those of succeed- 

 ing head somites. At a fourteen mm. stage the walls have again become 

 thickened. This is especially true of the median and posterior walls. 

 At a later stage (IG mm.) the cells are being freely proliferated into^ 

 the lumen of the somite, and the median thickening is very marked. 

 This proliferation continues, probably externally as well as internally, 

 for the outline of the somite becomes gradually indistinct. At a 

 19 mm. stage (Fig. 4, SA) the somite consists simply of a solid 

 mass of cells, gradually thinning out into the general mesenchymatous- 

 tissue. At a 26 mm. stage no trace of the somite can be seen. While,, 

 as Hoffmann says, no muscle fibres are formed by this somite, still, as 

 both Miss Piatt, 91, and Neal, 98, obserTed, the cells proliferated into 

 the cavity assume an elongate form. 



The anterior somite undergoes an interesting change in its position 

 relative to the first somite. Originally pressed against the anterior 

 walls of both the 1st and 2nd somites it comes, at a 12.5 mm. stage 

 (Fig. 1), to occupy a position lateral to somite I, and in the angle 

 between that somite and somite II. This seems to be due to the 

 great enlargement of the eye vesicle, and also to the forward growth of 

 somite I. Soon, however, the outpocketing from the posterior end of 

 somite I, which gives rise to the inferior oblique, appears, and this 

 ultimatel}^ grows nearly around the anterior somite, so that this later 

 somite occupies a deep depression in its wall (Figs. 2 and 3). 



ISTeal, 98, p. 227, found at a 65 segment stage processes apparently 

 extending from the ciliary ganglion to this somite. I have not been 

 able to find such processes of whose nervous character I was certain. 

 FiKST, OE Peemandibular Somite. — The epithelial walls of this 

 somite give rise to four of the six muscles of the adult eye. It is there- 

 fore preeminently the eye-muscle somite of the head. Balfour stated 

 that this cavity was cut off from the anterior end of the coelom by the 

 formation of the first gill cleft. Marshall also held that it was cut off 

 from tlie anterior end of the coelom, but that this took place inde- 

 pendently of the formation of the gill cleft. In Scyllium and Pris- 

 tiurus. Van Wijhe found that the cavity was never in other than poten- 

 tial connection with the primary coelom, arising independently of it 



