Arthur B. Lamb 197 



thickened edge seem also muscuhir. At a 27 mm. stage I have been 

 able to find only the very slightest remains of this muscle. Such a 

 rapid degeneration seems, however, improbable, and requires, I feel, 

 further investigation. 



The superior oblique muscle derived from this somite is innervated 

 in the adult by the trochlearis. This nerve, however, is the last cranial 

 nerve to be differentiated, not appearing until a 21-22 mm. stage (Fig. 

 5). At a 16 mm. stage, as several investigators have shown, the small 

 ramus ophthalmicus superficialis V. sends fibres to this somite. From 

 this it might be inferred that motor impulses were originally trans- 

 mitted to this somite by the 5th nerve. Neal considers this supposition 

 untenable, since in embryos of but 19 mm. length, consequently before 

 the appearance of the trochlearis, the ramus ophthalmicus superioris V. 

 shows no connection with the muscle. While unwilling to contradict 

 this latter statement and say that such a connection does exist, I should 

 be even more unwilling, because of the very close proximity of muscle 

 and nerve at 19-24 mm. stages, to say that such a connection does not 

 exist. 



Thikd, ok Htoid Somite. — This is the most posterior somite which 

 contributes to the musculature of the eye, giving rise to the external 

 rectus muscle. It is marked off from the rest of the body cavity merely 

 by constrictions before and behind, at a time Avhen the 7th somite of 

 Van Wijhe is completely separated. This emphasizes the progressive 

 development which takes place both forward and backward from a point 

 in the neck region. At this stage its form and position is very similar 

 to that of the four succeeding head somites. At a 22 segment stage a 

 contracted lumen is generally visible. ISTeal points out that at this 

 stage the somite is in every way comparable with a trunk myotome. 

 It is plainly dorsal in its topographical relations to the notochord, 

 dorsal aorta and dorsal wall of the alimentary canal. Mesenchymatous 

 cells are plainly proliferated from a well marked area on its median 

 wall, forming an outgrowth comparable to the sclerotome of trunk 

 somites. The somatic wall is plainly epithelial and there is a well 

 marked myocoel. Finally, the somite is, as will be seen later, inner- 

 vated by a nerve which is generally recognized as comparalile with the 

 ventral root of a spinal nerve. 



At a 28-30 segment stage this somite is completely separated from 

 its neighbors. Its lumen has become well marked, while anteriorly the 

 somite has assumed a bilobed appearance. At a 32 segment stage this 

 lobation is especially evident. More marked growth now takes place 

 in the middle and posterior part of the somite, and at the same time 

 the walls there begin to lose their epithelial character. 



