No. 3.] THE VERTEBRATE HEAD. 539 



the surface observations ; sections of the embryo illustrated 

 in Fig. 25 show that the mesoblast is nowhere divided into 

 segments, while the epiblastic somites extend throughout its 

 entire length. When the stage represented in Fig. 26 is 

 reached, the first mesoblastic somites (three in number) are 

 to be detected, in the trunk region ; the rest of the mesoblast 

 remains undivided, but the epiblastic segmentation is as before. 

 I have also made a careful study of the sections of the chicken 

 embryo illustrated in cut 7, and can find no segmental divisions 

 of the mesoblast outside the protovertebrae (five in number), 

 but the metameric divisions of the epiblast extend from the 

 extreme anterior tip back to the point where the neural folds 

 fade out. 



5. Presence of these Segments in Embryonic Rim and Primitive 



Groove. 



Another point to be noted regarding these primitive segments 

 is their range. They do not end at the posterior limit of the 

 axial embryo, but extend on either side into the embryonic rim. 

 As the embryo increases in length, those in the embryonic 

 rim are apparently drawn into the axial embryo ; at any rate, I 

 do not find evidence of extension by budding in the axial 

 embryo, and, in the early stages, the number of neural 

 segments therein increases in direct ratio to the lengthening 

 of the embryo. This would indicate that at least some of the 

 material of the embryonic rim contributes to the elongation of 

 the embryo. 



In the chick the walls of the primitive groove are also 

 divided into segments that are similar to those that appear in 

 the neural folds. 



6. Number of Segments represented in the Brain. 



There is considerable variance in the conclusion of different 

 observers as to the number of neural segments represented in 

 the brain. Orr, McClure, Waters, agree in identifying six in 

 the hind-brain of the chick and lizard, and five in the hind- 

 brain of Amblystoma. In estimating the entire number in 



