426 A Three Weeks' Iluniiin Embryo 



of the epidermic thickening. The thickened epidermis, separated from 

 the brain, extends cephalad from the point of contact toward the olfac- 

 tory region and hence away from the hypophysis (Fig. 7). Following 

 still farther away from the hypophysis the olfactory region is found 

 (Figs. 3, 4, 5), separated by a sharp fold from the eye-stalk. From the 

 above, the only logical conclusion seems to me that this specimen gives 

 positive evidence that the olfactory region of the brain is not its mor- 

 phologic cephalic part, but that the eyes are relatively to the original 

 margin of the neural plate cephalad of the olfactory region, i. e., nearer 

 the hypophysis. This is in contradistinction to the arrangement that has 

 always been accepted namely, that in the vertebrate brain the olfactory 

 region is the most cephalic, forming the first of the series. Even His 

 after his acceptance of the demonstration of Keibel and his own state- 

 ments concerning the neuropore ignored the logical conclusion as to the 

 order of the parts in the series. 



Following von Baer, Eeichert, and Gotte, Studnicka''' has finally de- 

 monstrated that the olfactory lobes and the cerebrum are essentially 

 dorsal and paired outgrowths from the neural tube. The present investi- 

 gation confirms this and further places the eye-stalk and the retina in a 

 similar category as dorsal paired organs serially in front, i. e., toward the 

 hypophysis from, the olfactory lobe. The natural corollary follows that 

 the optic chiasma crosses the original margin or dorsimesal line; that it 

 is in serial order with the pre-commissure, forni-commissure, callosum, 

 supra-commissure, and post-commissure, each binding together paired 

 dorsal organs, the chiasma being as truly dorsal as the post-commissure. 



Nor is the conclusion above reached based merely upon logic. The 

 studv of a model made of a mouse in which the neural plates are cleft to 

 the hypophysis show that in tracing the series of folds, which have rela- 

 tion to the margin, there are on each side: 1st, Hypophysial rudiments 

 both in the skin and the brain, consisting of folds which reach the margin ; 

 2d. A fold which ends in the outgrowing eye, and extends to the margin, 

 while along the outer surface of this fold the skin is in direct contact with 

 the brain; 3d, A flap or margin, still undifferentiated, lying between the 

 2d fold and the mesencephal. By comparison of this specimen with later 

 stages of various specimens, it is seen that the flap (3d) becomes differ- 

 entiated into olfactory, cerebral, and diencephalic rudiments. 



A model made of the neural plates of the human embryo 12 of the 

 Johns Hopkins University collection, lends strong confirmatory evidence 



^1 Studnicka, F. K., Kgl. bohm. Ges. Wiss., Math.-natur., XIV, 1901; Zool. 

 Centralbl., VIII, 1901. 



