260 



G. R. DE BEER 



rudiment by a certain distance, occupied by flat ectoderm ; 

 there is as yet no upper lip. 



In an embryo twelve days old from the time of fertilization 

 (Text-fig. 2), which has consequently hatched, the anterior 

 region of the brain has extended slightly and the antero-ventral 

 surface of the head lies practically horizontal. A depression 

 U-shaped in section near the anterior extremity is the invagina- 



Text-fig. 1. 



Petromyzon planeri. Embrj'o just prior to hatching, 

 rudiment of hypophysis and stomodaeum. 



AdO, adhesive organ. A.O., accessory organ. D.W., dorsal wall of 

 fore-gut. F.h., fore-brain. H.c, hypophysial cavity. H.d., 

 hypophysial depression. H.r., hypophysial rudiment. H.s,, 

 hypophysial strand. /., infundibulum. L.I., lateral lobes of 

 ' Uebergangsteil '. N., notochord. iV.c, cartilage of nasal 

 capsule. O.c/t.. optic chiasma. O.i., olfactorj' invagination. 

 0.11., olfactory nerve. P. A., pars anterior. P.E., pineal eye. 

 P.I., pars intermedia. P.N., pars nervosa. S.i.h., solid ingrowth 

 of the hypophysis. St., stomodaeum. T.c, trabecula cranii. 

 U., ' Uebergangsteil '. U.I., upper lip. 



tion of the olfactory organ. Farther back is the larger similarly 

 U-shaped invagination of the stomodaeum ; but between them 

 is a slight dent on the surface corresponding to a region where 

 ectoderm cells are pushing in beneath the brain. This is the 



