142 CHARLES EUGENE JOHNSON 



sorbed in the mesenchyma, seems probable from the facts in 

 the series which follow. 



The second head somite of the 7-mm. stage is of such indis- 

 tinct and indefinite form that it may easily escape notice. It 

 reaches here the most obscure phase of its development. The 

 more or less conspicuous cavities of earlier stages have collapsed 

 and broken down, and with their disappearance the cells of their 

 walls are with difficulty distinguished from the intruding and 

 intermingling mesenchymal elements. There is a tendency, how- 

 ever, for the cells bounding the cavities to form tiny scattered 

 clusters, and these, together with a few stringlets which are 

 unmistakable portions of the cavity walls, make it possible, 

 guided by its position with respect to the trigeminal ganglion, 

 to determine the approximate extent of the somite district. 



Processes similar to the above are taking place in the muscle 

 anlage of the mandibular arch immediately adjacent to the second 

 head somite, but the cavities in this area persist somewhat longer. 

 Nevertheless it is difficult at this stage to draw any sharp line 

 between the two districts. 



The third head somites in this embryo have essentially the 

 same appearance as in the 6-mm. series represented by figure 

 12. 



8-mm. embryo; sagittal series 



The most noticeable change in the first head somite of this 

 stage is that it has still further expanded so that the ventro- 

 lateral diverticulum has been practically obliterated in the pro- 

 cess, a shallow broad depression alone remaining. The somite 

 is now a thin-walled sac, flattened in the antero-posterior direc- 

 tion, and tapering rapidly medially into a conical hollow stalk. 

 There is no connection between the somites of the two sides. 

 The prechordal plate is no longer found, having been entirely 

 taken up into the stalk and medial walls of the somite. The 

 notochord is still more sharply bent back so that its tip seems to 

 be in actual contact with the oral entoderm a short distance 

 posterior to the hypophysis. A slight but distinct protrusion 

 of the dorsal entodermal wall occurs at this point. The tip of 

 the chorda bears a small group of cells which clearly differ from 



