156 CHARLES EUGENE JOHNSON 



lar mass on each side, in the interior of which cavities arise. 

 These cavities coalesce, and eventually a large thin-walled vesicle 

 is formed which reaches its maximum size at about the 9-mm. 

 stage. A constant feature of the development of this somite 

 is an extension from its ventro-lateral wall which enters into 

 close temporary association with the developing musculature of 

 the mandibular arch. 



The area of outgrowing cells on the wall of the foregut, from 

 which the first head somites are formed, very soon becomes 

 differentiated into a thick-walled epithehal body with slit-like 

 lumen — the so-called prechordal plate — which is connected later- 

 ally with the first head somites by a slender solid cell-stalk. 

 The notochord ends anteriorly in the posterior wall of the plate. 



As development proceeds, the cavity of the prechordal plate 

 enlarges and its walls become thinner, as some of its cells pass 

 over into the stalks of the somites. In some cases the cavities 

 of the somites push into the stalks and connect with the cavity 

 of the prechordal plate, thus forming a temporary connecting 

 canal between the two somites. But in other instances the stalk 

 of one or the other of the somites may become constricted off 

 from the prechordal plate at a relatively early stage, and whether 

 or not a cross-canal would in such cases be formed is difficult 

 to say without more extensive study. In Chelydra the cross- 

 canal, where found, is rather narrow. In some reptilian forms 

 it is very broad. A part of the prechordal plate may remain 

 attached to the end of the chorda and later become lost in the 

 mesenchyma. 



The second head somite. The second head somite arises in 

 the dorsal mesoderm at the side of the neural tube, just below 

 and slightly anterior to where the trigeminal ganglion later ap- 

 pears. It probably first appears as a small heap of concentrated 

 mesodermal cells, but the early phase of this somite in Chelydra 

 seems less clear than in Emys as described by Filatoff. These 

 cells then become arranged in a radial manner about a central 

 point or lumen, and assume the form of a small somite. The 

 somite very soon becomes expanded into a thin-walled vesicle 

 of more or less spherical form which may be accompanied by one 



