Humphrey, Brain of the Snapping Turtle. 93 



jal and Van Gehuchten, of Europe. These latter authors by 

 the use of the Golgi method have advanced our knowledge of 

 this subject until it is now nearly complete. Van Gehuchten 

 says that the Cajals have made a comparative study of the optic 

 lobes in all classes of vertebrates but unfortunately this has not 

 been published. 



The various authors differ widely in their interpretation of 

 the number of layers in the tectum opticum. Osborn describes 

 eight in the frog, Nakagawa (57) probably following him, finds 

 eight in Rana, Tropidonotus, Emys and Columba. Spitzka 

 finds seven, but his ental layer includes the two ental of 

 Nakagawa. I find these eight layers easy to recognize in Chel- 

 ydra. In the bird, Turner (87) figures thirteen, Ramon y Cajal 

 fifteen, Van Gehuchten (32) divides the tectum primarily into 

 three but subdivides these layers again. He bases his division 

 upon the probable function of the different parts. The ectal 

 layer consists of the fibers of the optic nerve, these, as noted 

 above, spread out over the tectum. The second layer includes 

 all the space between this layer and the inner fibrous layer near 

 the mesoccele (Fig. 25). The third is this inner fibrous layer. 

 This division of the layers has a physiological clearness and is 

 anatomically useful. The difference in the number of layers 

 given by careful observers shows that a definite number is 

 mostly a matter of personal preference. Figure 25 shows a 

 portion of a frontal section of the tectum. In it the arrange- 

 ment of the parts is figured. The three layers of Van Gehuch- 

 ten are shown and also the eight layers of Nakagawa. No dis- 

 tinct boundary can be drawn between layers. Cajal divides the 

 ectal layer of Van Gehuchten into seven and the middle layer 

 into six. Reference to Figure 25 shows that the ectal layer of 

 Van Gehuchten (A') corresponds with the ectal layer of Nak- 

 agawa (A). This layer consists of retinal fibers and a few scat- 

 tered cells. The middle layer of Van Gehuchten (B') includes 

 five of the layers of Nakagawa (B-F). The first four of these 

 layers have the same essential structure and are differentiated 

 only by the relative number of cells. These cells are oval, 

 rather irregularly scattered through a stroma consisting mainly 



