433 A Three Weeks' Human Embryo 



the roof of the dienccphal. In this specimen, at first glance, it seemed 

 that the only one from this region which could possibly extend to the roof 

 of the diencephal, is the albicantial. The early history of the fold as 

 seen above, does not make the interpretation seem probable and, moreover, 

 a careful modeling of the region as shown in Figs. 6, 8, 9, seems to indi- 

 cate that the albicantial and the diencephalic folds, originating at widely 

 separated parts of the brain in the middle line, end near each other in the 

 lateral wall but are distinctly not continuous. 



This albicantial fold is only tentatively put at the beginning of the 

 series, since from its original close approximation to entodermal rather 

 than ectodermal tissue, it does not seem to belong to a truly dorsal series 

 nor to be the ventral end of a diencephalic fold. 



1. Hypophysial. — These folds are strongly developed in later human 

 embryos. They are not sharply outlined in this specimen but the pair 

 can be distinguished lying opposite to the pair of widely open pouches 

 representing the hypophysis at this period (Fig. 9). There seems to be 

 no fact thus far found which might bar these folds from the series. In 

 a young mouse (see above), the folds as modeled show distinct relation 

 to the margin of the neural plate, while the associated organ, the hypo- 

 physis, is a really paired organ " from the ectoderm as distinguished from 

 the early entodermal relationship of the albicans. In this connection, 

 the position of the hypophysial as the first in the series, it is significant 

 that Boeke" finds in Amphioxus and certain fishes a ciliated pit in the 

 region of the infundibular process, having according tb him, the physical 

 appearance of an organ of sense. Should there be confirmation of this it 

 would represent a lost sense organ, the first of a dorsal series. 



2. Optic or Eye-stalk. — In its earlier stages, this region is represented 

 by a pair of wide furrows extending from the margin of the neural plate 

 to the pouches forming the optic vesicles. With the closure of the plates, 

 each furrow forms a wide vesicle connected in the present specimen, with 

 the epidermis through the neuropore (Figs. 4, 6, 8, 16). As development 

 proceeds, the portion of this vesicle toward the hypophysis, is constricted 

 by a complicated folding to form the so-called optic nerves, the portion 

 toward the olfactory remaining single as the pre-optic recess. 



S. Eye-vesicle or eye proper. — As shown in Figs, 6, 8, and 9, the eye 

 is distinctly constricted off from its stalk. In Figs. 6-8, it shows second- 

 ary folding, but a typical cupping does not occur. 



The eye-vesicle seems to have relationship with the original margin, 



=''Gaupp, E., Arch. f. mikr. Anat., XLII, 1S93. 

 " Boeke, J., Anat. Anz., XXI, 1902. 



