388 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in this case the supposition of a pro-oral intestine, and seems to arguo 

 against the theory that vertebrates once had a more primitive mouth 

 placed anteriorly to that which they now possess. Previous to the 

 development of any whito matter in the brain six symmetrical swellings, 

 separated by sharp lateral constrictions, appear in the hind-brain; these 

 arc homologous with the medullary folds found by Kuppfor in osseous 

 fishes ; in the lizard they give rise to the roots of tho fifth, sixth, com- 

 mon root of seventh and eighth, and roots of tho ninth and tenth nerves ; 

 it is proposed to call these swellings neuromeres. About the time of 

 disappearance of the neuromeres, nerve-fibres begin to appear as lateral 

 bauds of longitudinal fibres passing along tho lateral external surface of 

 the spinal cord and brain, and uniting in a single band on the morpho- 

 logically anterior surface of the brain immediately ventral to the optic 

 stalks. The fibres of tho optic nerve appear on the internal surface of 

 the eye-cup, and on the anterior surface of the hollow of the stalk ; they 

 differ from the fibres of all other nerve-roots in not being developed as 

 polar outgrowths of the cells. 



Shortly after the first appearance of lateral bands of fibres a second 

 system begins to developo as polar outgrowths of the cells lying just 

 internal to the lateral band ; they extend ventrally, and form a con- 

 tinuous ventral commissure, which ends at the point where the floor of 

 the midbrain merges into the infundibulum. About the same time tho 

 commissures of the anterior part of tho brain begin to develope ; they 

 are all morphologically dorsal, and are nearly similar in their develop- 

 ment. The author has obtained essentially the same result with 

 urodelous and anurous Batrachians. 



Development of Petromyzon.*— Dr. W. B. Scott, in the present 

 memoir, deals only with the development of the nervous system and 

 sensory organs of Petromyzon. He finds that the upper lip rotates 

 through an arc of 180°, and this has a great effect on the development 

 of the anterior organs of the head. In the freshly hatched larva the 

 brain, and especially the fore- and mid-parts, are exceedingly small, in 

 correlation, no doubt, with the undeveloped condition of the sense- 

 organs during the greater part of larval life. The cranial flexure is 

 always slight, and is partly corrected by a rotation in the opposite 

 sense. The hemispheres arise as an unpaired solid mass, and the 

 olfactory lobes are formed from them. The infundibulum is a diverti- 

 culum of the thalamencephalon, which is at first single, but afterwards 

 divides into lobus and saccus ; the epiphysis arises as in other verte- 

 brates, and soon exhibits its character as an optic vesicle, but has no 

 lens ; the primary gives rise to a secondary vesicle which enters into 

 close relations with the left ganglion habenulsB ; the changes in 

 characters and relations which it undergoes suggest that it has acquired 

 some secondary character of importance, but what that is cannot be 

 guessed. The right ganglion habenulse is from the first much larger 

 than the left ; the former comes to project above the roof of the brain, 

 and the latter divides into two portions which are connected by a fibrous 

 stalk. 



The pituitary body is derived from the epiblast of the surface of the 

 head in close connection with the olfactory involution ; Dr. Scott be- 

 lieves that this connection is secondary ; the morphology of this structure 



* Joum. of Morphology, i. (1887) pp. 253-310 (4 pis.). 



