38 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



the rest of the forebrain. The optic evagination does not reach quite to the ectoderm and 

 there is no evidence of any lens thickening. 



The second forms the cerebral hemispheres. Above and behind the origin of the optic 

 evagination, and involving the roof of the forebrain, is a small but distinct expansion, 

 which is the beginning of the cerebral hemispheres. No evidence of the olfactory lobes are 

 to be seen yet in front of the cerebral lobes, and this is probably to be associated with the 

 presence of the still open neuropore, as this lobe is demonstrated by Mrs. Gage (1905) in 

 a somewhat older embryo where the neuropore is closed. 



The third division of the forebrain corresponds with what Mrs. Gage calls the infun- 

 dibular fold. It is in exactly the same position as figured by her and is distinctly cut off by 

 furrows from adjacent parts. It is situated immediately below and extends behind the 

 optic evagination, involving thus the floor and lower part of the lateral wall of the forebrain. 

 Also involving the floor is the fourth fold, the hypophyseal, forming that part of the 

 pituitary body derived from the brain. It occurs immediately in front of the cephalic 

 flexure and is thus the furthest caudal portion of the floor of the forebrain. It is a distinct 

 median evagination containing a small tubular lumen and projects directly caudad, to lie 

 just in front of the anterior end of the notochord (plate 1) and against the front end of the 

 gut. No pouch of Rathke yet reaches it. 



The fifth fold in the forebrain differs from the condition found by Mrs. Gage in this 

 respect, that one fold here extends from the posterior part of the roof of the forebrain, 

 obliquely downward and forward across the lateral wall, ending just above the hypophyseal 

 and infundibular folds. This includes all the region which is found in the albicantial and 

 the two diencephalic folds in the embryo described by Mrs. Gage. This fold is large and 

 well marked — in fact is more developed than any other except the optic. 



The midbrain (plate 1 and plate 3, fig. 5) is quite distinctly marked out and is quite in 

 accord with the descriptions of it as found in other young embryos. It occurs at the region 

 of the cephalic flexure and so is wedge-shaped, the apex being the floor or ventral surface 

 and the base of the wedge being the dorsal surface of the brain. The midbrain is incom- 

 pletely subdivided into two portions, an anterior and posterior on each side, by a shallow 

 groove starting on the roof and passing half way down the sides, where it is lost. The folds 

 formed thus foreshadow the formation of the corpora quadrigemina. In Mrs. Gage's 

 case this division of the mesencephalon is much more marked. 



The isthmus is not well marked and no constriction or lessened development are in 

 evidence at this stage, as shown by the diameters. 



The hindbrain (plate 1 and plate 3, fig. 5) or rhombencephalon is well developed and 

 shows a marked degree of differentiation for an embryo so young, all the total folds 

 described by Broman and by Mrs. Gage being readily identified. Four cranial ganglia on 

 each side are also to be found, representing 5 nerves, the trigeminal, facial, auditory, glosso- 

 pharyngeal, and vagus. 



Of the cerebellum there is little evidence yet, that part of the hindbrain from which it 

 arises being hardly differentiated. The fourth ventricle, however, is indicated, for the 

 dorsal part of the lumen, especially in the posterior portion, is expanded, appearing lozenge- 

 shaped on section, while the ventral part of the lumen is cleft-like throughout. 



On the side wall of the rhombencephalon and involving the floor are found 7 neuro- 

 meres (plate 3, fig. 5) which agree accurately with those found in this region of the brain 

 of early human embryos by Mrs. Gage and by Broman. 



