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 habenulce. 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 



