HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 137 



following posterior portion, and is somewhat wavy in its course, 

 giving the impression that it is preparing to disintegrate. Such, 

 however, is not the case at this time. It is still attached to 

 the prechordal plate. In figure 8 the group of cells near the 

 prechordal plate is a part of this section of the chorda. 



5-mm. embryo (20 segments); sagittal series; reconstruction: 



figure 19 



On the right side of this embryo the first head somite is a large 

 smooth-walled vesicle, flattened somewhat in an antero-posterior 

 direction, and tapering rapidly medially where its wall passes 

 into a short thin cell-strand (connecting-stalk), ending close to 

 the ventro-medial side of the prechordal plate. Within the 

 median half of the vesicle a slender cell-band extends from the 

 anterior to the posterior wall, being the only remains of the 

 earlier solid mass of the interior. In the ventro-lateral wall of 

 the somite the hollow process of the younger stage is represented 

 by a prominent diverticulum {dv.), which tapers down into a 

 short loose strand and becomes lost in the more extensive and 

 conspicuous cell-clusters in the mesenchyma which give rise to 

 the maxillo-mandibular musculature. 



The left somite is essentially like the right but somewhat more 

 expanded. It is also connected, through its stalk, with the 

 prechordal plate. A solid portion of the stalk next to the latter 

 structure shuts off communication between their cavities. 



Measurements of the prechordal plate at this time show that 

 it is very considerably reduced, as compared with the 4-mm. 

 stage, in all its dimensions. This reduction is the result of the 

 drawing in of its cells into the somite wall by way of the median 

 stalk. 



The second head somite of this stage lies somewhat laterad, 

 between the ophthalmic and maxillo-mandibular divisions of the 

 N. trigeminus. Its vesicles here reach their maximum size. 

 As seen in the model the right somite consists of two main lobes 

 or vesicles: a larger dorsal lobe, and a smaller ventral one. The 

 cavities of the two connect by a narrow canal. The dorsal lobe 



