138 CHARLES EUGENE JOHNSON 



is itself divided into two nearly equal lobes by an indentation 

 on its lateral side. The ventral vesicle is intimately associated 

 with the muscle anlage of the mandibular arch, in the same way 

 as the diverticulum of the first head somite. 



The muscle anlage of the mandibular arch is here differentiated 

 into an irregular, rather extensive dorsal part, and an elongate 

 narrow ventral portion leading down through the arch, in a 

 ventro-medial direction, as far as the pericardium. This ventral 

 portion is now a very well differentiated district in the mesen- 

 chyma, and its cells are deeply stained and densely packed; but 

 here and there throughout its extent a number of open spaces 

 occur. The dorsal portion is looser in structure and is less sharply 

 marked off from the surrounding mesenchyma. It seems a com- 

 mon characteristic of the cells of these districts to first form small 

 •solid spherules, and then develop cavities, a process quite similar, 

 on a minute scale, to that occurring in a somite. 



On the left side of the embryo the second head somite forms 

 a single large cavity, the longer axis of which is caudo-ventral. 

 No connection with the cavity of the third head somite can be 

 made out, but its dorso-medial wall is in contact with the anterior 

 vesicular portion, of the latter. Its relations to the mandibular 

 arch musculature are the same. 



The third head somite, like the second, in this stage reaches 

 the m.aximum development of its cavity-phase, but it is a smaller 

 structure. As a whole this somite is elongated, reaching from 

 a short distance anterior to the facio-acustic ganglion to the 

 dorso-medial wall of the second head somite. The anterior divi- 

 sion is a thin-walled sac which is fused on its ventro-lateral side 

 with the dorso-medial wall of the second somite. A broad canal 

 connects the cavities of the two. In the posterior division of 

 the somite the cells have for the most part lost their former 

 more definite epithehal order, retaining it only on a part of the 

 median wall. They appear greatly increased in number, are 

 less sharply limited at the periphery, and loosely surround a 

 relatively small cavity which opens abruptly into the larger 

 cavity of the anterior division. In form this part of the somite 

 is more elongate than formerly. The anterior division of the 



