434 CHARLES HILL, 



as described by Locy, but denies their segmental importance. He was 

 unable to find them in stages immediately preceding closure of the neural 

 groove, and bases a conclusion on this negative evidence, together with 

 their irregularity, that the early segments have no connection with those 

 of later stages. The evidence in Teleosts and chicks is very clear as to 

 their unbroken continuity, and I know also from my own observation 

 that corresponding segments are present in Acanthias where Neal failed 

 to find them. The view first advanced by Locy that these early 

 segments are the same as the neuromeres observed in the medulla 

 from 1828 onwards, receives support from the work recorded above. 

 And it also strengthens the contention that in the early stages 

 a jointed condition is present in the anterior portion of the neural 

 axis that agrees in all essential features with that observed at the 

 same time in the region of the medulla. 



The anterior segments precede the large divisions known as fore-, 

 mid- and hind-brains. The failure to recognize this fact has led to 

 confusion and conflicting reports as to the segments in front of the 

 cerebellum , notably these larger divisions have , from time to 

 time, been made homologous with the segmental divisions of the 

 medulla. The fact that these larger divisions later subdivide, before 

 the segments of the medulla have disappeared, has led to the er- 

 roneous view that the fore- and mid-brains are morphologically equi- 

 valent to the segments of the medulla (Neal and Miss Platt). 



As appears above (page 419, Figs. 32—40 r) the primary fore- 

 brain, in chick embryos in later stages, is divided into prosencephalon 

 and thalamencephalon by a dorsal constriction that is very different 

 from the transverse grooves that separate adjacent segments in the 

 medulla. In still older stages another transverse dorsal constriction 

 (s) appears which divides the thalamencephalon. To these divisions 

 Waters and Zimmermann appear to have assigned the same seg- 

 mental value as to the segments of the medulla. It seems to me this 

 is clearly an error as is shown not only by the late appearance of 

 these anterior divisions, but, also, by the developmental history of the 

 dorsal grooves that produce these divisions (see page 419—421). Her- 

 rick very appropriately writes: "It is scarcely legitimate to count 

 dorsal diverticles like those of the fore-brain with ventral expansions 

 like those of the mid-brain. Waters and others seem to have made 

 this mistake" (in: Journ. comp. Neurol., 1892, p. 168). 



Since the head is a segmental structure, how may the history 

 of its segments be read ? The raesoblastic divisions (head cavities) have 



