367 
on account of the optic expansion. Specialization and rapid expansion 
to form the large fore- and mid-brain vesicles appear to be factors 
in obliterating the first five primary segments. 
Figures 19 and 20 represent the neural segments in embryos 
respectively 50 and 80 hours old. The segments of the hind-brain 
(6—11) are still present and represent the well known neuromeres of 
Orr, Mc CLureE and others. It should be borne in mind that these 
six segments are identical with those designated by the same numbers 
in the earliest stages. The feature of difference is the obliteration of 
the first five segments through the very rapid growth of the walls 
of the fore- and mid-brains. 
A dorsal constriction (7) has appeared dividing the fore-brain 
into prosencephalon and thalamencephalon. A second dorsal groove 
(s) divides the latter into nearly equal parts, an anterior and a 
posterior portion. The fact that these constrictions appear so late in 
the ontogeny and are preceded by segments entirely similar to those 
in the medulla renders it very improbable that the two set have the 
same morphological value. 
Study of Sections, Chick and Trout. 
As my series of sections agree completely with the surface studies 
the following description will be brief. 
In the Chick, after the neural groove has closed, all the cha- 
racteristics of a typical “neuromere”, as defined by Orr ’87, are 
present (Fig. 21). The fundamental value of these characteristics will 
be discussed in the complete paper. An interesting fact to be observed 
here is that transverse grooves mark the limits of neural segments 
on both the external and the internal surface of the brain. This is 
true of both the Chick and the Trout embryo (Figs. 21 and 22). In 
the former, in later stages, the segmental grooves are elevated on 
ridges of the internal surface of the brain. In the Trout no such 
elevations occur. With this qualification there is complete harmony 
as to the number, origin and developmental history of the neural 
segments in the Trout and the Chick. 
As to nerve relations, we must remember that internal origin and 
various components are the essential point but it will be convenient 
to locate their superficial union with particular segments, in order to 
establish more accurately the identity of the neuromeres here described 
with those of other authors. 
The olfactory and optic nerves are connected respectively with 
the first and second segments. From the ventral region of the mid- 
