136 CHARLES EUGENE JOHNSON 



detached vesicles on its postero-dorsal wall. There is no connec- 

 tion on this side between the cavities of the second and third 

 head somites, though their walls are closely approximated. Fig- 

 ure 7 is taken from a second embryo of the same age, and it 

 shows the transition stage between the third head somite of the 

 4-mm. embryo and that of a 5-mm. specimen presently to be 

 described. It shows clearly the transformation of a typical 

 somite into a large thin-walled vesicle such as, in all head somites, 

 forms the most striking phase. 



Jf.B-mm. embryo; transverse series: figures 8 and 9 



A number of sections of this series in the region of the second 

 and third head somites were broken in mounting, and thus ren- 

 dered unreliable for observation on those structures; but the 

 region of the first head somites is well preserved, and here con- 

 siderable advances over the preceding stage have taken place. 

 The dorsal parts of these two somites have expanded into large 

 thin-walled cavities, triangular in section, with their bases in 

 close proximity to the optic vesicles and their apices approaching 

 each other near the midline. Further ventrally the large cavity 

 ends, and as seen in sections passing through the prechordal 

 plate, the structure becomes more irregular and consists of a 

 dense cell-mass in which a number of larger and smaller cavities 

 are forming and coalescing, whereby the main cavity becomes 

 gradually extended ventrally and medially into the connecting- 

 stalk (fig. 8). On one side only, the left, is the somite connected 

 with the prechordal plate (fig. 9) . The plate itself shows decided 

 change. It is reduced in actual size, its cavity is larger, and the 

 walls are thinner and looser. On one side the cells of its wall 

 continue into a short, more or less tubular stalk, the wall of 

 which then expands laterally into the head somite. There is 

 no connection as yet between the lumen of the prechordal plate 

 and the cavities in the stalk. On the opposite side the connecting- 

 stalk ends freely a short distance laterad of the prechordal plate. 



The short flexed portion of the notochord next to the pre- 

 chordal plate is peculiar in that it is more slender than the 



