HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 139 



somite lies snugly in the angle formed by the ophthalmic and 

 maxillo-mandibular divisions of the N. trigeminus. The trunk 

 of the maxillo-mandibular division lies close against the outer 

 wall of the somite, between its anterior and posterior divisions. 

 On the left side conditions are similar. The anterior division 

 of the somite seems somewhat more expanded, and on its ventro- 

 lateral side there is a small detached vesicle lying close against 

 its wall. In the posterior division an irregular cavity remains 

 within the loose cell-mass. Between the two portions of the 

 somite is a loose cellular partition, but no open connection be- 

 tween the enclosed cavities is found. 



6-mm. embryo; sagittal series: figures 10 to 12 



The first head somite of this embryo differs from that in the 

 preceding specimen chiefly in its more expanded condition. In 

 its lateral portion a horizontal septum gives it a two-chambered 

 appearance. On each side the prominent diverticulum of the 

 preceding stage is represented, reaching down to meet an exten- 

 sion of the maxillo-mandibular muscle-mass. This mass or an- 

 lage here presents a very sharply defined form throughout most 

 of its extent, being least definite in its dorsal extension towards 

 the first and second head somites. Its long ventral portion, 

 extending down through the mandibular arch, is a thick-walled 

 tubular mass very suggestive of the visceral portion of the man- 

 dibular somite in elasmobranch embryos. Dorsally it divides 

 into two short diverging processes, one directed anteriorly (fig. 

 10, pr.) to meet the diverticulum from the first head somite, the 

 other extending towards the second head somite, becoming irregu- 

 lar and indefinite in its upper portion, where it spreads into a 

 diffused area of small cavities and solid cell-clusters. 



The prechordal plate is further reduced in size, having been 

 gradually drawn into the median connecting-stalk of the left 

 somite, with which alone it is connected. On account of the 

 increased expansion of the head cavity, the stalk has' become' 

 shorter proportionately, and now forms a short, rapidly tapering 

 tube on the inner ventral wall of the somite. The somite cavity 

 extends into its base, but the apical portion is solid and fused 



