140 CHARLES EUGENE JOHNSON 



with the prechordal plate. On the opposite side the somite has, 

 as before noted, no connection with the prechordal plate. 



The rate at which the prechordal plate becomes incorporated 

 into the stalk of the first head somite seems to vary. In another 

 embryo of apparently precisely the same age, the plate is of 

 considerably larger size, lying as a short transverse tube, with 

 compact epithelial walls, against the end of the chorda. On the 

 right side it tapers into a slender cell-cord which then enlarges 

 into the somite stalk.. On the left a broader connection exists. 

 The wall of the prechordal plate continues directly into the tubu- 

 lar stalk of that side, and its lumen is confluent with the cavity 

 of the latter. Thus it is evident that the prechordal plate be- 

 comes embodied in the stalks of the first head somites and there- 

 fore really forms a part of their walls. It is not equally divided 

 between the two, but may be taken up in greater part by either 

 the right or the left, as the case may be. 



The second head somites in two series of the stage here con- 

 cerned show essentially like conditions, but minor variations 

 appear in each. Figure 11 is chosen from the second of the two 

 series, and here the somite is represented by a larger bilobed 

 vesicle which has a smaller simple one lying against its anterior 

 wall. The bilobed form of the larger vesicle is caused by the 

 deep indentation of its lateral wall. This somite is the simplest 

 of the mandibular somites in the two embryos, since it has the 

 smallest number of lobes and independent cavities. In the 

 others a greater number of cavities occur but they are of smaller 

 size. In all cases, however, the cavities of the second head somite 

 are sufficiently closely grouped so that they may readily be dis- 

 tinguished as a whole from the similar though generally smaller 

 cavities of the mandibular arch. 



The third head somite is represented by a large mass of cells 

 occupying most of the space between the trigeminal and facio- 

 acustic ganglia (fig. 12). The roof of the hyomandibular cleft 

 approaches it closely from below. A large, densely packed mass 

 forms its posterior division, and from the anterior side of this • 

 a narrow depressed cell-heap extends forward, closely hugging 

 the ventro-medial side of the maxillo-mandibular division of the 



