SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 161 



larval life. This is closely correlated with the limited activities and 

 sluggish life of the larvae (p. 224). 



The structures in the roof of the diencephalon are earlier to de- 

 velop than those of the mesencephalon. In the 26-day larva the 

 epiphysial vesicles and ganglia habenulae are already well developed. 

 It is probable that the elements from the right habenula which con- 

 tribute to the formation of Keissner's fibre have at this time sent 

 their- axons into the ventricle, though the evidence on this point is 

 not conclusive. 



The tectal reflex cells ai'e the first elements to develop in the mesen- 

 cephalon. In their early development tliey are aggregated in the 

 median plane immediately behind the posterior commissure, but later, 

 owing to the development of surrounding and intervening tissues, they 

 are divided into two lateral groups dorsal to the commissure. 



The axons of the cells passing into the ventricle coalesce, forming a 

 rmmber of trunks, which in their course through the mesocoele remain 

 distinct or but loosely aggregated until a late stage of larval develop- 

 ment. At first the fibre passes through the canal from the third to 

 the fourth ventricle free, but during the second month the rapidly 

 developing tissues of the meso-metencephalic fold enclose and surround 

 the fibre so that it becomes for a portion of its course embedded in the 

 nerve tissue. Meanwhile, daring the first month, the posterior canal- 

 cells have developed, and having established connection with the cord 

 by dendrites, send their axons cephalad through the canal. The con- 

 nection between these two intra-canal systems is not established until 

 the second month. 



In the larval Petromyzon the condition of the apparatus is very simi- 

 lar to that in Amia, but with the increase in morphological complexity 

 of the adult brain this resemblance disappears. In the larvae of the 

 first month the characteristic ependymal thickening previously described 

 cannot be distinguished. In advanced larvae, thirty days old, the de- 

 veloping ependymal cells in the lateral grooves ventral to the posterior 

 commissure are more compact and take the stain somewhat more 

 deeply (Plate 1, Fig. 4). The ependymal grooves in the adult are 

 conspicuously developed on both sides of the median plane, extending 

 around the fold of the posterior commissure into the prolongations of 

 the mesocoelic recess, and on the right side continue cephalad to the 

 base of the right ganglion habenula. The constituent axons of Keiss- 

 ner's fibre pass between the cylindrical ependymal cells. of the grooves 

 before entering the ventricle. The fact that this compact ependyma is 



