Johnston, The Brain of Petromyzoti. 73 



Acipenser, especially as regards those elements which are in 

 relation with the optic fibers. The question suggest itself, is 

 the tectum relatively primitive or reduced, undeveloped or de- 

 generated ? The fact that the paired eyes present undoubted 

 primitive characters indicates that the tectum is primitive. This 

 accords with the description of the morphology of the cells 

 related to the optic fibers. The writer hopes soon to investi- 

 gate the brain of Bdellostoma with reference to this point. 



Note — The paper of Scott ('87) on the development of Petromyzon escaped 

 my attention when this paper was being written. As it contains several items of 

 interest, a note is inserted here in the proof. ScoTT describes and figures the' 

 part played by the growth of the upper lip in carrying the olfactory pit up on 

 the dorsal surface of the head. During this process, he says, the cerebral flexure 

 partly corrects itself by rotation about a transverse axis through the mid brain. 

 That this straightening of the brain is due to the pressure from the upper lip is 

 not stated by Scott, hut seems probable. This, together with the telescoping 

 movement of the fore brain suggested in the present paper will account for the 

 definitive form. 



Sco IT has not traced the course of development of the body which he calls 

 the second epiphysial vesicle (paraphysis of Retzius ei al.), but he states in his 

 summary that it is derived from the first vesicle (epiphysis)- and comes into 

 close relation with the left ganglion habenulae. The latter ganglion, in larvae of 

 12—25 millimeters, elongites and divides into two parts. The anterior part, per- 

 haps during metamorphosis, becomes closely associated with the lower pineal 

 vesicle and connected with the left ganglion habenulae by a fiber tract. This 

 account of the origin of the mass of cells in the lower wall of the so-called 

 paraphysis leaves no doubt as to their nervous character, but it throws no light 

 upon the origin and homology of the vesicle itself. It is altogether probable 

 that this is not a true paraphysis, since the paraphysis is a fold of the choroid 

 plexus. Further investigation of the morphology, histology, and physiology of 

 this region in Petromyzon is much needed. 



ScoTT also states that the N. lateralis develops by a progressive differentia- 

 tion of the ectoblast, with which the account of v. Kui'FFER is in agreement. 



C. Primitive Characters in the Petromyzon Brain. 



The brain of Petromyzon shows its primitive character 

 least in the general relations of its chief centers and fiber tracts 

 and most in the characters of its nerve elements and the fine 

 structure of the various centers. In the former regard this brain 

 is much like the characteristic fish brain ; in the latter respects 

 it shows great peculiarities. In the fine structure of many parts 

 of the brain there is a marked simplicity as compared with the 



