Developmental history of primary segments of the vertebrate head. 431 



mining this number. From the fact that he thinks, for some reasons, 

 segmentation is retarded cephalad to the cerebellum, it seems probable 

 that he has also, as McClure, observed the late secondar}^ division 

 that appears in the primary fore-brain, as represented in Figs. 40 and 41. 



LocY, '95, from a surface study of undissected chick embryos 

 has observed IIV2 segments in the dorsal region of the neural groove 

 cephalad to the first formed somites. My description of the neural 

 axis of the dissected embryonic chick as embodied in this paper, is a 

 verification of these observations. 1 am also indebted to Dr. Locy 

 for placing at my disposal an unpublished manuscript with figures, 

 prepared some years ago by Mr. F. A. Hayner, one of his students, 

 who for more than a year made a close study of the early neural 

 segmentation in the embryonic chick finding at the close of the first 

 day of incubation 11 V2 neural segments in front of the first formed 

 protovertebra. 



According to my observations there is complete agreement as to 

 the number and position of the neural segments in front of the cere- 

 bellum in the trout and the chick. The fore-brain has 3 and the 

 mid-brain has 2. These segments do not differ in the earliest stages 

 in any essential features from those of the medulla. They are present 

 in the same early embryonic stages as those of the medulla but owing 

 to the rapid changes that take place in the anterior cephalic region 

 they disappear very early, those of the fore-brain first and those of 

 the mid-brain second. They antedate the historic divisions, fore- and 

 mid-brains, and precede even the optic evaginations. In short they 

 represent a phylogenetic condition older than the organs, possibly 

 even older than the phylogeny of the neural axis. 



Value of Various Segmental Criteria, 



a) Mesomeres. My observations were naturally directed to 

 the joints of the neural axis because, in the head region, they afford 

 the most evident traces of joints or segments to be found ; but, the 

 mesoblast of the head region was also subjected to close scrutiny. It 

 was hoped that at least some trace of the head cavities that are 

 present in the cephalic mesoderm of the shark, would be found in the 

 Teleost embryo. The study of numerous sections of both chick and 

 trout embryos has given no positive evidence of the presence of such 

 cavities. The loose and somewhat scattered mesoderm occasionally 

 presents a grouping of cells that might be interpreted of segmental 

 significance but otherwise this tissue remains unbroken. 



