HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 135 



rounding mesenchyma, and delicate cytoplasmic processes bridge 

 the intervening spaces. 



The third head somite also presents marked differences. In- 

 stead of the single cavity of the 3.5-mm. stage there lies in its 

 place a compound structure having the appearance of a somite 

 which is about to be divided by a circular constriction into two 

 parts (fig. 6). Each part has a relatively large and distinct 

 lumen, separated entirely from that of the other by. a rather 

 thin partition of cells pushed inward from the wall of the somite. 

 The N. trigeminus passes obliquely outward in close proximity 

 to the dorsal wall of the anterior division. 



A closer examination of the two parts of this somite shows a 

 number of differences between them. The two are of unequal 

 size. In the posterior division the wall is thicker, and its cells 

 are arranged as a well-defined epithelium about the lumen. This 

 part of the somite also lies further mediad. On the anterior 

 side its wall becomes noticeably thinner, beyond the constric- 

 tion, as it there passes directly into the wall of the anterior divi- 

 sion. The cavity of the latter is somewhat larger. This latter 

 portion of the somite is in the first stages of expansion into the 

 so-called 'cavity' phase. Its wall has come into contact and 

 fused with the median wall of the second head somite, and a 

 narrow canal leads from one into the other. This close associa- 

 tion of the second and third head somites is the direct result of 

 the expansion of the two structures, especially of the former. 



In the present stage it can be seen that a portion of the meso- 

 derm, extending from just below the outer edge of the second 

 head somite down to where it passes over into the pericardium, 

 has become more densely packed; and immediately ventrad of 

 this somite and adjacent to the ventro-lateral diverticulum of 

 the first head cavity, small clusters of mesodermal cells appear, 

 varying in size, in part solid, and in part enclosing cavities. This 

 entire differentiated area forms the anlage of the musculature 

 of the mandibular arch. 



The opposite side of the embryo shows conditions essentially 

 similar, but the second head cavity is more irregular and is flat- 

 tened somewhat in the antero-posterior direction. It has two 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 14, NO. 2 



