saegent: the optic eeflex apparatus of vertebrates. 149 



I 



B. Observatioks. 



My studies in this group have been based on series of adult brains of 

 Petromyzon mai'iuus sectioned in transverse and sagittal planes. My 

 investigations on the development of the optic reflex apparatus in 

 cyclostomes are incomplete, and because of lack of material, have been 

 confined to larval stages of Petromyzon planeri twenty-six and thirty 

 days after hatching. The larvae at this stage are from 6 to 10 mm. 

 in length. 



1. Development of the Apparatus ix Petromyzon. In the 26- 

 day larvae the brain vesicles are still of large size, and the differ- 

 entiation of the structures in the walls of the mesencephalon is yet 

 incomplete. The roof in the median plane is very thin, being made 

 up for tlie most part of a single layer of cells (Plate 1, Fig. 1). The 

 region of the posterior commissure is beginning to be downfolded, and 

 the fibres of the commissure have begun to make their way from side 

 to side. The structures of the diencephalon are much more advanced. 

 The ganglia habenulae with their fibre tracts are well developed and 

 project downward into the diacoele (Plate 1, Fig. 4). The riglit gan- 

 glion is already of much greater size than the left. I have, however, 

 been unable positively to identify at this stage in the right babenula the 

 constituents which in the adult contribute to the formation of Reissner's 

 fibre. 



a. Optic Reflex Cells. Immediately posterior of the downward 

 flexure through which the posterior commissure is to pass there is a 

 group of cells larger than the surrounding neuroblasts, from which they 

 have apparently become differentiated. These cells lie on either side of 

 the median plane, and by their position and size may be recognized 

 as the tectal reflex cells. By developing processes they are becoming 

 multipolar. A few of them have at this early stage already begun to 

 send out their axons obliquely into the ventricle (Fig. 1, cl. opji. rfx.). 



In the 30-day stage (Fig. 2) the optic reflex cells have increased in 

 number and size. They lie along the median plane in the roof of the 

 optic lobe, close to the posterior commissure. In sagittal sections the 

 cells in the anterior portion of the tectum are seen forming a single or 

 double layer. In a transverse section just behind the posterior com- 

 missure these cells at this stage form a ridge projecting into the meso- 

 coele (Fig. 3). In later stages, as the surrounding structures develop, 

 the relations of these cells are greatly changed. The axons emerge 

 into the ventricle in or near the median plane, so that in a strictly 



