No. 2.] DEVELOPMENT OF PETROMYZON, 299 



afterwards divides, though remaining soh"d ; the lateral ventricles 

 first appear in larvae of about 14 mm. length. The olfactory- 

 lobes are formed from the hemispheres, not from special pro- 

 liferations of the olfactory epithelium. 



(5.) The infundibulum is formed as a diverticulum of the 

 floor of the thalamencephalon, and is at first single, though in 

 small larvae it soon divides into lobus and saccus. 



(6.) The epiphysis arises as in other vertebrates ; it soon 

 exhibits its character as an optic vesicle, though vv^ithout a lens ; 

 a second vesicle is formed from the primary one, which enters 

 into intimate relations with the left ganglion habenulae. Its posi- 

 tion is at first median, but the growth of the right ganglion 

 habenulae forces its point of attachment over to the left side. 

 The acquirement of some secondary function by the epiphysis 

 is probable. 



(7.) The ganglia habenulae are from the first unsymmetrical, 

 the right being much the larger. The right shifts its point of 

 attachment somewhat, and projects above the roof of the brain; 

 the left divides into two portions connected by a fibrous stalk. 



(8.) The pituitary body is derived from the epiblast of the 

 surface of the head in close connection with the olfactory involu- 

 tion. This connection is regarded as secondary, and reasons are 

 given for the belief that this organ is the rudiment of a canal 

 once opening on the surface of the head. 



(9.) The mid-brain has a considerable extent of floor. The 

 optic lobes do not appear till late in larval life in correlation with 

 the retarded development of the eyes. 



(10.) The hind-brain undergoes few changes; the principal 

 being the thinning of the roof, formation of the choroid plexus, 

 and breaking up of the cell-mass into nuclei. The cerebellum 

 is formed from the posterior wall* of the dorsal fold between the 

 mid and hind-brains ; it long remains very minute. 



(11,) The spinal cord in the later embryonic and early lar- 

 val stages is like that of the higher vertebrates : the characteris- 

 tic flattening takes place in the larva. 



(12.) The peripheral nerves are developed much as in the 

 Selachians ; the olfactory nerves are originally paired, the gan- 

 glion is derived from the olfactory epithelium, though not as 

 Shipley has described ; the optic nerves are remarkable for their 

 great length at first ; the ciliary ganglion and its root arise from 



