HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 157 



or two much smaller vesicles. These may arise independently 

 from smaller somite-like structures, as described in connection 

 with the 3.5-mm. embryo, or by a budding-off process from the 

 main cavity. The cells forming the wall of the vesicle or vesi- 

 cles, constitute the somite. The vesicle reaches its maximum 

 expansion at about the 5-mm. stage and may still be accompanied 

 by a small number of minor cavities. A single main cavity is 

 always present. 



At this stage the ventral side of the somite is in close contact 

 with an area of small cell-clusters and minute cavities, which 

 have arisen in the mesoderm of the mandibular arch, just ventrad 

 of the second head somite. They extend ventro-medially through 

 the arch to the pericardial region, and form the anlage of the 

 maxillo-mandibular musculature. From now on, the somite 

 suffers a collapse of its walls and becomes broken down into a 

 considerable number of smaller cavities, which in turn are reduced, 

 until eventually but a mass of mesenchymal cells remains, scarcely 

 distinguishable from the surrounding mesenchyma. 



The third head somite, like the second, arises in the dorsal meso- 

 derm close to the ventro-lateral side of the hind-brain, between 

 the location of the facio-acustic and trigeminal ganglia, but 

 nearer the latter. In an embryo of 3.5 mm. the third head somite 

 is a well differentiated structure with clear cut lumen and well 

 defined epithehal walls; and it is slightly compressed, so that 

 its principal axis is longitudinal. A httle later this somite is 

 subdivided, but at the stage under discussion, painstaking study 

 of the sections, supplemented by a carefully made wax recon- 

 struction failed to reveal any indication of subdivision. The 

 subdivided condition, therefore, is to be considered as secondary, 

 and results from the division of a primarily undivided somite. 



The anterior division of the somite rapidly becomes expanded 

 into a rather large vesicle, and for a time it may be connected 

 with the second head cavity through a fusion of a part of their 

 contiguous walls. The posterior portion is of considerably greater 

 mass, has thicker walls and expands but sUghtly, so that its 

 cavity is relatively small. It is connected for a short time with 

 the cavity of the anterior division, and the walls of the two are 



