322 Robert J. Terry. 



in length, a stage in which the diencephalon and telencephalon are 

 demarcated. The names proposed for them, taken in order from 

 before backward, are: 



1. Paraphysal arch 4. Superior commissure 



2. Velum tr&nsversum 5. Epiphysis 



3. Post-velar arch * 6. Posterior commissure. 



According to the author, ''The homologues of all these parts, 

 exist probably in all vertebrates." 



Other questions suggested and discussed in the paper concern 

 the distinction between the paraphysis and paraphyseal arch, the 

 genesis of the choroid plexus, the significance of the great difference 

 of development of the post- velar arch among animals, the relations 

 of the superior commissure and the position of the posterior com- 

 missure with reference to the diencephalon and mesencephalon. 



Following the direction of research given in Minot's paper, 

 Dexter ('02) and Warren ('05), in investigations of the avian, 

 amphibian and reptilian brains, have supported the view of the 

 general occurrence of the primitive subdivisions and also have 

 noted especially the distinction between the paraphyseal arch 

 and organ. 



The present study ^ was undertaken with the object of extending 

 the same line of inquiry to the teleostean brain. In view of the 

 differences in the mode of formation of the medullary tube in the 

 elasmobranchs and teleosts, misgivings were felt at the start that 

 the early form of the fore-brain roof would necessitate an inter- 

 pretation of the value of its parts in terms of the elasmobranch 

 type rather than present parts easily identified and directly com- 

 parable with the divisions of the shark's brain. To some extent 

 this was true, but the early appearance of the epiphysis, posterior 

 commissure and velum was sufficient to make certain the interpre- 

 tation of the remaining regions. The method of study adopted 



^ This investigation was made during the year 1906-7 while the writer held an 

 Austin Teaching Fellowship in the Department of Histology and Embryology in 

 the Harvard Medical School. For the opportunities there afforded for anatomical 

 research and especially for the encouragement received from the director of the 

 department, Professor Minot, he is deeply giateful. 



