182 bulletin: museum of compaeative zoology. 



off from the almost spherical cell on the side nearest the third ventricle ; 

 it passes directly toward the ventricle, into which it projects, growing 

 into the cerebro-spinal fluid. During the second day of larval devel- 

 opment the axons may be seen projecting into the ventricle from the 

 anterior portion of the optic tectum and torus, resembling cilia in their 

 length and appearance. 



Early in the third day the adjacent axons, projecting into the meso- 

 coele, come together in groups of three to six, and begin to coalesce, 

 forming apparently a single fibre. At a later stage in the third day 

 the fibres formed by these united axons have grown farther into the 

 ventricle, converging and coalescing as they grovv backwards, and thus 

 form the fibre of Reissner (Plate 4, Fig. 'Ao). At first these elements 

 of Reissner's fibre are but loosely aggregated, often appearing as a 

 confused, tangled mass of fine fibrils, but the union becomes more 

 complete as the fibre grows caudad and development proceeds. 



In Amia of the fourth day the region of the optic lobes has advanced 

 rapidly in development and attained the condition typical of larval life. 

 The posterior commissure is well established, and its development is 

 already producing a downward growth in the median plane (Plate 4, 

 Fig. 25, cd'ms.p.). The tectum opticum has attained greater thickness 

 and the cells are now more closely aggregated at its anterior end in 

 the median plane, but extend posteriorly rather irregularly and at greater 

 intervals through the tectum on either side of the median plane, showing 

 some tendency posteriorly to group themselves in 3's and 4's. The cells 

 have increased slightly in size, but there has been no further increase in 

 number. The system of axons in the optic ventricle is still diffuse at 

 the anterior end, and even as far back as the cerebellum, but more 

 compact posteriorly. Each axon emerges into the ventricle separately, 

 most of them in or near the median plane, the system showing to best 

 advantage in median sagittal sections. As the tectum has developed in 

 thickness by the multiplication of surrounding neuroblasts, many of the 

 cells have been withdrawn, so to speak, from immediate proximity to 

 the ventricle by the development of intervening tissue, and their axons 

 have taken a course through the tectum toward the recess above the 

 posterior commissure, wliere near the median plane the axons emerge 

 into the ventricle (Plate 5, Figs. 36, 37). 



By the sixth or seventh day (Fig. 35), this w^hole apparatus has 

 attained the condition typical of larval life, and later in the adult 

 undergoes only slight modification. In the development of the tectum 

 opticum and posterior commissure, the position of the cells and their 



