130 CHARLES EUGENE JOHNSON 



zoDtal to the region in front of the hindbrain. In carefully exam- 

 ining the anterior portion of this series, beginning with the first 

 sections which are dorsal, and proceeding ventrally, there appears 

 a section at a level slightly above the floor of the mid-brain and 

 passing through both optic evaginations, in which a group of 

 closely packed cells is seen lying in the mesoderm at the side of 

 the neural tube, opposite the constriction between the dienceph- 

 alon and the mesencephalon. Following this cell-group four 

 sections further ventrally it appears as a well-defined, though 

 rather small, structure in which the cells are arranged radially 

 about a central point; their nuclei are more deeply stained than 

 those in the surrounding mesoderm and lie toward the periph- 

 ery. In some parts of the structure there seem to be two or 

 three irregular layers of cells, in other parts but one. In the 

 next section (fig. 1) there appears what seems to be a narrow 

 sht-like cavity which can be traced through only two sections. 

 This feature can be made out only with high power (about 300 

 diameters), but the entire body is readily observed with low 

 power (65 to 80 diameters). Four sections further ventrally the 

 Umit of the structure is reached. It thus extends through a 

 total of six 8-micron sections. Frequent mitotic figures occur 

 throughout. 



Separated from this structure by not more than two sections, 

 a second group of cells appears, smaller than the first, but show- 

 ing in two consecutive sections a similar radial arrangement of 

 the nuclei about a central clearer protoplasmic area, with doubt- 

 ful traces of a lumen. Beyond this group, wliich extends through 

 four sections, no further differentiations in the mesoderm can 

 be seen in this region. 



Through the hind-brain region, the plane of section falls at 

 right angles to the neural tube. The notochord follows the 

 flexure of the tube and is sharply bent so that its anterior end 

 is seen in horizontal section, lying slightly separated from the 

 ventral brain wall. On each side, near the ventro-lateral wall 

 of the hind-brain and at the level of the chordal flexure, a sharply 

 differentiated body appears in the dorsal mesoderm, which resem- 

 bles a typical somite (fig. 2). Each of the bodies consists of a 



