232 ORR. " [Vol. I. 



cling constriction in the region c. The condition of the cranial 

 flexure is here well illustrated. In the curved floor of the mid- 

 brain {MB) there are peculiar transverse folds disappearing up 

 the sides of the brain, — as though the curvature of the brain 

 had compressed its floor into folds. This seems to preclude the 

 hypothesis that the curvature is caused by a lack of growth in 

 the floor of the brain. 



In the middle of the anterior wall of the fore-brain, between 

 the optic stalks, is a small transverse groove (optic groove, og). 

 It has a very slight lateral extension, appearing only in about 

 four sections. The position of the groove between the optic 

 stalks seems to correspond to the anterior end of the slit above 

 referred to, and to the dorsal edge of the anterior fold. Though 

 I have not been able to trace out exactly and prove such an 

 origin for the groove, yet I think there is little ro.om for doubt- 

 ing that it is the last trace of the union of the lateral medullary 

 folds, above the anterior fold. There seems to be no other 

 plausible explanation for the presence of such a groove in this 

 particular spot. This groove also appears in amphibian 

 embryos. Goette has pictured it in Bombinator, and I have 

 observed it in the frog and Amblystoma. In a median vertical 

 section of an embryo of Amblystoma, just ventral to the 

 groove begins an enlarged thickening of the brain-wall, which 

 extends toward the hypophysis, and there thins out into the 

 thin floor of the brain in the infundibular region. The posi- 

 tion of this thickening in relation to the hypophysis charac- 

 terizes it as the anterior fold. I cannot determine the relation, 

 in this embryo, of the groove to the closure of the brain, as the 

 latter has already taken place. 



The eye is the first of the sense organs to develop, the second 

 and third being respectively the ear and olfactory organs. The 

 term " outgrowth " hardly conveys the full significance of the 

 first appearance of the eye rudiment. In Fig. 45 it will be seen 

 that the walls of the optic vesicles are parts of the lateral brain- 

 wall, which remain in their primitive contact with the epiblast, 

 while the other parts have sunk inward from the epiblast. This 

 inward-sinking is continued until every part of the central 

 nervous system is separated from the epidermis by the meso- 

 blast, and only the eyes retain their original contact. At a later 

 period mesoblastic tissue enters the eye-cup by the choroid slit, 



