108 THE DEVELOPMENT OF THE CHICK 
brain. The first clear landmark is given by the mesoblastic so- 
mites, because it is known that the four anterior somites are 
cephalic. All of the neural tube in front of the fifth somite is 
therefore cranial. What a large proportion of the neural tube 
this is in early stages may be seen by comparison of figures of 
embryos in the period covered by the chapter (cf. Fig. 61). Be- 
fore the appearance of the first somite the entire medullary plate 
in front of the primitive streak is in fact cranial. 
Origin of the Primary Divisions of the Embryonic Brain. The 
embryonic brain is divided into three divisions of unequal length, 
viz., the fore-brain (prosencephalon), mid-brain (mesencephalon), 
and hind-brain (rhombencephalon). The first division is character- 
ized in the period we are considering by its very considerable 
lateral expansions, the rudiments of the optic vesicles (Figs. 59, 
61, 63, ete.), and also by the fact that there is a suture in the 
anterior portion of its floor owing to the mode of its origin (Tig. 
62). A definite constriction between it and the following division 
first appears in embryos with six or seven somites (Fig. 59). At 
the stage of 9-10 somites the next division (mid-brain) becomes 
clearly marked off by a constriction from the hind-brain (Fig. 
61). The latter is relatively very long, and its anterior half is 
characterized in the 12-somite stage by the existence of five divi- 
sions (neuromeres) separated by constrictions (Fig. 63). 
It will be noted that the first neuromere of the hind-brain appears 
about twice as large as the succeeding ones; it really includes two neuro- 
meres according to some authors. Similarly, it is maintained that the 
mid-brain includes two neuromeres and the fore-brain three. 
According to Hill’s account the entire brain of the embryo chick 
is composed of eleven neuromeres or neural segments, which are formed 
even in the 1s stage. The first three enter into the composition of the 
fore-brain; the next two, viz., 4 and 5, form the mid-brain, and the last 
six the hind-brain. 
The three that enter into the composition of the primary fore-brain 
have the following fate according to Hill: the first forms the telen- 
cephalon, the second the anterior division (parencephalon) and the third 
the posterior division (synencephalon) of the diencephalon. The cere- 
bellum arises from the first neuromere of the hind-brain, sixth of the 
series. This question is more fully discussed in Chapter VI. (See 
Fig. 83.) 
