PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 461 
to determine the total number of segments or neuromeres, and 
especially to clear up the doubt in which my predecessors 
have left the mid-brain. The form studied was Gadus 
morrhua, embryos from six to eleven days’ incubation being 
used, for which I am greatly indebted both to Mr. H. V. 
Wilson, of the United States Fish Commission Station at 
Wood’s Holl, and to Dr. William Libbey, of Princeton; embryos 
of Amblystoma punctatum were also used by way of com- 
parison. I greatly regret that my efforts to obtain the 
embryos of Petromyzon were without success, as my obser- 
vations are thus limited to Teleostean forms. However, the 
results obtained from the investigation of young stages of 
the lamprey will, I hope, form a second part of this paper. 
Tn the early stages of the Cod the small amount of cranial 
flexure renders a horizontal longitudinal section of the entire 
neuron possible, and in such sections the walls of the brain 
are closely approximated, there being no proper lumen anterior 
to the fourth ventricle, its course and extent being indicated 
merely by the cell nuclei. The brain in section through the 
ventral portion, i.e. below the forming cerebral hemispheres 
and cerebellum, shows a perfectly straight and narrow tube, 
the walls of which are in apposition but not fused, as the line 
of their demarcation is perfectly apparent. In the earliest 
stages examined (about six days) the optic diverticula were 
well formed, as was also the auditory pit. The side walls of 
the fore- and mid-brain regions, however, were perfectly 
regular in extent, and showed no traces of neuromeric seg- 
mentation (see fig. 1), This I found to be true of all stages 
under ten days’ incubation, at least as far as the fore- and 
mid-brain regions are concerned, certain of the sections show- 
ing some of the characteristic hind-brain neuromeres. This 
fact is difficult to explain if, as seems probable, these seg- 
mentations are the remains, in part atavistic, of a primitive 
condition, on any other ground than that of the gradual abor- 
tion and disappearance of these structures. McClure has 
referred to the degeneration of the neuromeres, and it seems 
to me not unreasonable to conjecture that these constrictions, 
