CYCLOI'IA IN THE HUMAN EMBRYO. 17 



The lateral view of the head as given in the illustrations on plate 1 may be 

 compared with a normal embryo of the same size; for this purpose I will take 

 the Huber embryo No. 3, pictured by Streeter in figure 86, Volume II, of the Manual 

 of Human Embryology. In the Huber specimen the floor of the forebrain is un- 

 usually large, as may be observed by comparing the above-named figure (86) 

 with Streeter's figure 83, which is taken from His's embryo Br 3 . This region is 

 also somewhat smaller in the Huber embryo than in a model of the brain of our 

 No. 163 made by Dr. Lewis. No. 163 is 9 mm. long, and a profile drawing of its 

 brain is shown in figure 428 in the second volume of the Manual. Careful com- 

 parison of the model of the brain of the cyclopean embryo No. 559 with models 

 of the brain of normal embryos shows clearly that there is a decided ventral median 

 defect of the brain of this embryo, from the mammillary bodies to the front end of 

 the brain. This defect naturally takes away the tissues between the eyes and 

 between the cerebral vesicles. In other words, the floor-plate, as shown in figure 

 55 in the Manual, reaching from the mammillary bodies to the neuropore, has been 

 cut out. In the cyclopean specimen the hypophysis is also absent. The eye- 

 stems have been taken out, the olfactory lobes are absent, and the brain is reduced 

 to a single vesicle, as is the case in older specimens of cyclopia. Such an extreme 

 destruction of the base of the brain rarely occurs in cyclopean Fundulus embryos, 

 as in this animal, according to Stockard and Lewis, the brain is frequently entirely 

 normal, the eyes alone being deformed; but in man marked brain defects have 

 always been found to accompany cases of cyclopia. 



The optic vesicles in No. 559 form an hourglass-shaped body with two lenses, 

 as shown in plate 1, figure 1, and plate 3, figure 4. The tissues are beautifully 

 preserved and apparently normal in structure. The primary chambers of the 

 eyes communicate freely with each other (plate 3, fig. 4), and through a common 

 eye-stalk, which in turn communicates with the ventricle of the brain (plate 3, 

 fig. 2). The tapetum nigrum covers the optic vesicles and crosses the midline on 

 the dorsal side of the eyes that is, between the eyes and the common cerebral 

 vesicle. The tapetum stops abruptly at the optic stem and passes down slightly 

 along the anterior wall which joins the two retinas. The choroidal fissures reach 

 clear through the front of the eyes, running almost together as they approach the 

 common eye-stem, as shown in plate 1, figure 1. The topography of the optic 

 stem, ganglion layer of the retina, and tapetum is in the order given, starting from 

 the midline. in the normal embryo, as is probably the relation of their primor- 

 dia in the normal neural plate, judging by the work of Eycleshymer and of Lewis. 

 Eycleshymer showed that very early in development the eye appears, in amphibia, 

 as two pigmented spots lying quite close to the midline in the anterior end of the 

 medullary plate. If these groups of pigmented cells are destined to form the 

 tapetum, then the ganglion layer of the retinas would form nearer the midline, 

 while the cells which cross the midline would probably form the optic stem. 

 That Eycleshymer's view is correct is indicated by the work of Locy in his studies 

 upon the shark and by Keibel in his studies upon the pig. According to these 

 authors, the eye primordia arise from small depressions near the midline of the 



