134 CHARLES EUGENE JOHNSON 



'Zwischenplatte' of Filatoff. The notochord enters its posterior 

 wall, making it appear as though its tip were expanded into the 

 thick-walled prechordal plate. 



Owing to the increasing size of the fore-brain the first head 

 somites have been pushed back so as to lie nearly at right angles 

 to the notochord. They are still much flattened in the antero- 

 posterior direction. On the right of the embryo the somite is 

 attached to the anterior end of the prechordal plate by a narrow 

 stalk of cells proceeding from its ventro-medial surface; the 

 somite of the opposite side presents a similar short stalk, but is 

 completely separated from the prechordal plate. In sagittal 

 sections passing through the middle of these somites, each appears 

 as a short crescent, convex posteriorly. There is a relatively 

 large cavity in the dorsal horn, which, followed laterally, branches 

 into two narrower cavities. From the latero-posterior side of 

 the somite a rather slender cellular process extends a short dis- 

 tance caudo-ventrad into a denser portion of the mesoderm of 

 the mandibular arch, which is directly continuous ventrad with 

 the pericardial mesoderm. A cavity is present in the distal end 

 of this process. At this point it will be seen later that the first 

 head somite becomes closely associated with a number of smaller 

 cavities and cell-clusters appearing in the condensed mesoderm 

 area of the mandibular arch, which give rise to the maxillo- 

 mandibular musculature. 



The second head somite has undergone a marked change. 

 In exactly the same position occupied by the two somite-like 

 structures of the foregoing stage there here appears a very large, 

 somewhat globular vesicle with a smaller, roughly oval, vesicular 

 appendage on its antero-dorsal wall (fig. 5). The cavities of 

 the two are not continuous. Their respective positions do not 

 point to a formation from two such components as found in the 

 earlier embryos. Individually the cells of the wall do not differ 

 from the surrounding mesenchymal cells, yet, when the structure 

 is viewed as a whole, their close order around the sharply limited 

 lumen, and the numerous deeper-staining nuclei readily dis- 

 tinguish this cavity or somite from other mesenchymal spaces. 

 In many places the wall is distinctly drawn away from the sur- 



