264 OUTLINES OF CHORDATE DEVELOPMENT 



In the vicinity of the head process (Fig. 101), the vertebral 

 plates at this stage rapidly lose their definite character and the 

 cells scatter throughout the region, combining with the cells 

 ^ g .i continually being budded off from the walls 

 of the fore-gut, to form the general mesen- 

 chyme of the head region. Later scattered 

 cells are added to the mesenchyme directly 

 from the ectoderm of the head region. The 

 history of the mesoderm farther back, just 

 in front of the end of the primitive streak, 

 is much the same as in the body region 

 proper, although all of the embryo thus 

 far developed out of the primitive streak 

 shares in the formation of the head only. 

 Here the vertebral plate thickens still more 

 and its cells become rearranged so as to 

 form a short transverse break in the con- 

 tinuity of the plate (Fig. 100). On each 

 side the cells immediately in front of this 

 become grouped in a definite mass and 

 form the first pair of mesodermal somites, 

 continuous anteriorly with the forward ex- 

 tensions of the vertebral plates (Figs. 100, 

 104). This rearrangement of cells con- 

 tinues posteriorly, and soon a second split 

 appears on each side, a short distance be- 

 hind the first, cutting out a definite block 

 of the cells of the vertebral plate. This 

 is the second pair of somites. Additional 

 pairs of somites are blocked out in regular 

 fashion, as the embryonic region grows at 

 J; 5 1 the expense of the primitive streak. The 

 2^^ formation of somites is not actually com- 



O 



2 2 -g pleted until about the fifth day of develop- 

 .5 1 ment, by which time about fifty-two pairs 

 have been formed. The first somites formed remain the most 

 anterior in position. The first four pairs are ultimately in- 



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