EXPLANATION OF PLATES. 



Abbreviations. 



cbl., 



cl. opt. rfx., 



corns, p., 



e'phy., 



Cerebellum. 

 Optic reflex cells. 

 Posterior commissure. 

 Epiphysis. 



fbr. R., Reissner's fibre. 

 tct. opt., Tectum opticum. 

 vnt. III., Third ventricle. 

 vnt. IV., Fourth ventricle. 



PLATE 1. 



Figure 1. Amia calva, 1st day. Sagittal section (anterior end at right) through 

 roof of mid-brain. The large cells in the tectum opticum are the 

 tectal reflex cells, a few of which are just beginning to send out 

 their axons. 



Figure 2. Amia calva, 12th day. Sagittal section (anterior end at left) through 

 anterior part of tectum opticum. The numerous fibrils entering the 

 third ventricle are the axons of tectal reflex cells, a few of which 

 are shown in the section, but the most of which are lateral to this 

 section. 



Figure 3. Amia calva, 6th day. Transverse section of the spinal cord through 

 the ventriculus terminalis ; posterior canal cell sending its dendrites 

 through the fluid of the canal into the tissue of the cord. 



Figure 4. Amia calva, 1st day. Sagittal section through the posterior end of 

 the spinal cord and ventriculus terminalis, showing a single posterior 

 canal cell, its long process directed cephalad. 



Figure 5. Amia calva, 12th day. Sagittal section of a portion of the spinal cord 

 near its posterior end ; posterior canal cells in the ventriculus ter- 

 minalis and canalis centralis, sending their axons forward to form 

 Reissner's fibre. Reconstruction drawing from four successive 

 sections. 



Figure 6. Squalus acanthias, embryo 2 cm. long. Longitudinal sagittal section. 

 Posterior canal cells in the canalis centralis sending dendrites into 

 the cord and its axon cephalad. 



Figure 7. Petromyzon marinus, 30 days. Sagittal sections of the tectum opticum 

 (anterior end to the right) ; the axons of the developing tectal reflex 

 cells are just emerging into the third ventricle. 



PLATE 2. 



Figure 8. Amia calva, 12th day. Sagittal section through brain, somewhat 

 diagrammatic. 



Figure 9. Raja erinacea, 14 cm. long. Sagittal section of the mid-brain, dia- 

 grammatic. 



