22 CYCLOPIA INJTHE^HUMAN EMBRYO. 



tube pretty well closed up, leaving a large neuropore in the front. The Bremer 

 specimen is slightly in advance of this. We have also in our collection a similar 

 embryo (No. 470) in which the brain form corresponds exactly with that in the 

 specimens of Wallin and of Bremer. 



Furthermore, the twin specimens of Watt, which contain 17 or 18 somites, 

 correspond very closely with the three above-named embryos. In the Watt 

 specimen the neuropore is nearly closed and the eye-vesicles reach all the way 

 across the front of the brain-tube. The Pfannenstiel III embryo and the Bremer 

 embryo are found pictured by Bach and Seefelder, both in profile and in sections, 

 but I do not think we should unreservedly accept their description of the begin- 

 ning of the eye-vesicles as final for the human embryo. 



It seems to me that our knowledge of the form of the forebrain before closure 

 of the neuropore is much in need of revision, and towards this revision we have 

 assembled several new models of young embryos. The first is a model by Dr. 

 Bartelmez of our No. 1201 (University of Chicago, No. 87) from an embryo with 

 8 pairs of somites, which seems to me to bear very much upon this question. The 

 neural plate flanges out into a large tongue with a slightly hourglass-shaped de- 

 pression running across the midventral line (plate 2, fig. 6.) The larger lateral 

 depressions no doubt indicate the foveola, and the groove connecting them across 

 the midline is in the position in which the optic stalk develops later on. It would 

 probably be better if we accepted Froriep's designation of torus opticus for this 

 connection. The torus opticus seems very insignificant in this specimen, as 

 shown in the illustration (t. o.), but when we consider to what extent the torus 

 may be stretched, as illustrated by Bach and Seef elder (fig. 2, plate 13), we recog- 

 nize the importance of this structure. I am inclined to believe that the form of 

 the brain in the Bartelmez embryo must be viewed as normal, as it corresponds so 

 well in a series with that of several older human embryos, namely, those recently 

 described by Wallin and by Bremer and our No. 470. 



There is another ridge already indicated in the brain-tube of His's speci- 

 men EB reaching across the midline just below the neuropore. This is the 

 torus transversus of Kupffer; and those who are interested in this structure are 

 referred for greater details to the articles by Tandler, Kupffer, and Johnston. 

 The neuropore is found just closed in our embryos No. 148, published by Mrs. 

 Gage, and No. 836, which has been modeled by Dr. Evans, as well as a specimen 

 of the same size modeled by Johnston. Dr. Johnston has been good enough to 

 send me photographs of his model, so that I am able to compare it with our own. 

 It is clear to me that the large "optic vesicle," extending over the whole lateral 

 wall of the front part of the neural tube, represents more than the optic vesicle, 

 as it must also include the primordium of the cerebral hemispheres, since the torus 

 transversus touches the lower border of the neuropore and the optic vesicles fall 

 below this line. This large so-called optic vesicle must resolve itself into the 

 optic vesicle and brain hemisphere in subsequent development. In this process 

 the torus opticus gradually must become more pronounced. 



