YOUNG TWIN HUMAN EMBRYOS WITH [7-19 PAIRED SOMITES. 41 



however, does not exhibit quite as well developed a neural crest and series of spinal ganglia. 

 It does possess trigeminal, acusticofacial, and glossopharyngeal ganglia of approximately 



equal development to its twin, and also exhibits the extremely large extension of the facial 

 ganglia into the second gill arch. No vagus ganglion can be distinguished in Embryo V, 

 and it will be remembered that this ganglion in Embryo VI was small, and though quite 

 recognizable would easily be overlooked without careful search, so that this difference is 

 not of great significance. The only other difference worth mentioning is that shown in the 

 combined fold representing the albicantial and diencephalic folds as described by .Mrs. 

 Gage. As in Embryo VI, one fold only is found, in place of the three Mrs. Gage mentions, 

 and this fold is most prominent. It stands out in a winged fashion on each side of the fore- 

 brain, and is even much more conspicuous than in its twin. Its posterior boundary is 

 formed by a very deep groove, and this is the main factor in making the fold so prominent, 

 as the posterior or caudal wall stands out perpendicularly to the wall of the rest of the 

 forebrain. This fold is not so sharply delimited in front, because from the crest of the fold 

 the anterior or cephalic wall slopes gently forward and is much longer than the other. 



Apart from the differences mentioned above, there is complete agreement between the 

 stage of development and the appearance of the nervous system of these two embryos, and 

 no further description is necessary for the second. 



Regarding the question as to what point is the morphological anterior end of the brain, 

 I find valuable evidence here in support of the views advanced by Johnston (1910) that it 

 is situated at the recessus prseopticus. For a discussion of the various views on this ques- 

 tion I refer the reader to the descriptions of embryos by Airs. Gage (1905) and by Van den 

 Broek (1911). It is sufficient to state that His (1893) and others have placed the extreme 

 cephalic end of the ventrimesal line or basilar axis of the brain at a point on the anterior 

 wall of the recessus infundibuli. The dorsimesal line is marked, of course, by the fusion 

 of the lips of the neural groove. According to His, the side walls of the brain unite in front, 

 forming a closing seam between the ends of the dorsimesal and ventrimesal lines. This 

 occurs by the closure of the anterior neuropore. Most investigators have discarded the 

 view of His regarding this closing seam and Mrs. Gage says "it logically follows the cephalic 

 end is the point where the dorsimesal and ventrimesal lines meet." All writers seem to 

 agree that this point does not coincide with the location of the anterior neuropore and of 

 the part of it which longest retains its connection with the ectoderm. Stress is laid, how- 

 ever, on the presence of a seam as one means of identifying the dorsal surface. Now. in 

 both of the embryos here described, the nervous system is nowhere separated yet dorsally 

 from the ectoderm, so that there is no doubt of the location and extent of the dorsal seam. 

 At the anterior end of the body the anterior neuropore lying between the optic vesicles is 

 still wide open and is, as shown by the rolling of its lips out into the adjacent ectoderm, the 

 still open portion of the dorsal seam. Nowhere have I seen any account of the closure of 

 the nervous system in which it is stated to begin in any place except over the hindbrain 

 and upper part of the spinal cord, whence it extends both caudad and cephalad. No closure 

 begins at the cephalic end of the brain to meet the main closure, although the anterior 

 neuropore does not always close regularly from behind forward. It is reasonable, however, 

 to take the extreme cephalic end of the neuropore as the extreme front end of the dorsi- 

 mesal line. This point, then, will mark the extreme, front end also of the ventrimesal line 

 or basilar axis, and it is situated in front of the origin of the optic vesicles and forms the 

 preoptic recess. According to His and to Mrs. Gage and others, that part of the brain 

 wall from the preoptic recess back to the infundibular recess is dorsal. If so. in these 



