170 bulletin: museum of comparative ZOOLOCxY. 



one series of Raja erinacea, shortly after hatching, the fascicles, united 

 for the most part in one trunk, emerge into the recess above the 

 commissure. At the point of emergence the ependymal thickening is 

 continued outward over the fibre some little distance in a cone-shaped 

 prominence (Plate 11, Fig. 71). Some fine branches emerge into the 

 recess through the ependyma at otlier points, uniting Avith the fibre of 

 lieissner in the ventricle. In another series of the adult brain some 

 fine branches of Reissner's fibre are to be found extending through 

 the narrow I'ecess into the ventricle (Fig. 16). Some fine branches are 

 also found beneath the pars intercalatus and apparently come from the 

 habenular region. 



All these branches unite in the antei'ior portion of the niesocoele, form- 

 ing Reissner's fibre, which runs caudad through the ventricles and into 

 the central canal. It lies near the floor of the ventricles and follows a 

 straight course through them. In young individuals of R. erinacea its 

 diameter is from 2 to 3 micra ; in the adult it may have a diameter as 

 great as 6 or 7 micra. 



2. Squalus acanthias and Mustelus cams. a. Development. A 

 series of Squalus acanthias, from embryos 10 mm. long to the adult, has 

 been studied. The optic reflex apparatus is late in development. In 

 embryos from 10 to 30 mm. in length, no trace of the fibre of Reissner is 

 to be found, and the cells of the optic tectum are as yet undifferentiated. 

 The first evidence of a differentiation of the 'Dachkern' cells from the 

 neuroblasts of the optic tectum was observed in au embryo 35 mm. long. 

 Certain of the neuroblasts lying close to the ventricle in tlie median 

 portion of the tectum, and destined to form the 'Dachkern' cells, are 

 then distinguishable from the surrounding cells. In later stages (40-50 

 mm. long), in which the layers of the tectum are differentiated, these 

 cells increase in size and conspicuousness, staining more deeply than the 

 surrounding cells, but no distinguishable processes have yet formed 

 (Fig. \l,nidl.tct.). In the * pup ' just before birth the cells and pro- 

 cesses are well developed, and have attained nearly the adult condition. 



In embryos from 30 to 50 mm. in length, in which the 'Dachkern' has 

 not yet developed, there may be found a delicate fibre running through 

 the ventricles and into the centi-al canal (Plate 3, Fig. 17). This has 

 been traced forward under the * Schaltstiick,' and it probably arises 

 from the ganglia habenulae. It doubtless consists of those axo)is which 

 arise from the habenulae and join the fibre of Reissner. As the embryos 

 of Squalus are retained within the body of the mother until they reach 

 a length of 15 to 20 centimetres, the visual centres are retarded in de- 



