48 THE ORIGIN OF VERTEBRATES 
as it grows, increase in size uniformly, but a constriction appears on 
its dorsal surface at one particular place, so as to divide it into an 
anterior and posterior vesicle; then the latter becomes divided into 
two portions by a second constriction. In this way three cerebral 
vesicles are formed; these three primary cerebral vesicles indicate 
the region of the fore-brain, mid-brain, and hind-brain respectively. 
Subsequently the first cerebral vesicle becomes divided into two to 
form the prosencephalon and thalamencephalon, while the third 
cerebral vesicle is also divided into two to form the region of the 
cerebellum and medulla oblongata. 
These constrictions are in the position of commissural bands of 
nervous matter; of these the limiting nervous strands between the 
thalamencephalon and mesencephalon and between the mesencephalon 
and the hind-brain are of primary importance. The first of these 
commissural bands is in the position of the posterior commissure 
connecting the two optic thalami. In close connection with this are 
found, on the mid-dorsal region, the two pineal eyes with their optic 
ganglia, the so-called ganglia habenule. From these ganglia a 
peculiar tract of fibre, known as Meynert’s bundle, passes on each 
side to the ventral infra-infundibular portion of the brain. In other 
words, the first constriction of the dilated tube is due to the presence 
and growth of nervous material in connection with the median pineal 
eyes. Here in precisely the same spot, as will be fully explained 
in the next chapter, there existed in the arthropod ancestor a pair 
of median eyes situated dorsally to the cephalic stomach, the pre- 
existence of which explains the reason for the first constriction. 
The second primary constriction separating the mid-brain from 
the hind-brain is still more interesting, for it is coincident with the 
position of the trochlear or fourth cranial nerve. In all vertebrates 
without exception this nerve takes an extraordinary course ; all other 
nerves, whether cranial or spinal, pass ventralwards to reach their 
destination. This nerve passes dorsalwards, crosses its fellow mid- 
dorsally in the valve of Vieussens, where the roof of the brain is 
thin, and then passes out to supply the superior oblique muscle of the 
eye of the opposite side. The two nerves form an arch constricting 
the dilated tube at this place. In the lowest vertebrate (Ammoccetes) 
the constriction formed by this nerve-pair is evident not only in the 
embryonic condition as in other vertebrates, but during the whole 
larval stage. As Fig. 20, A and B, shows, the whole of the dorsal 
