CYCLOPIA IN THE HUMAN EMBRYO. 11 



appearance of the optic vesicle from the brain. In other words, the several degrees 

 of the cyclopean eye come off from the brain in their final condition. 



The idea of the fusion of the eye parts, Stockard continues, was deep-rooted, 

 however, and exists in the recent views of Spemann in a refined form. Spemann 

 believes, as others have previously suggested, that cyclopia is due to an absence 

 of non-ophthalmic tissue in the median region of the medullary plate or groove. 

 This lack of median tissue allows the eye primordia, which he holds to be lateral 

 in position, near the borders of the medullary plate, to come together and fuse in 

 the median plane and later give rise to a cyclopean eye. Cyclopia, according to 

 this view, occurs in a more or less passive manner, and is actually a fusion of the 

 eye primordia of the two sides during development. Stockard adds that he is 

 certain that this fusion explanation, which has now been forced entirely back 

 into the medullary plate, is as false as its bolder predecessor, which assumed the 

 fusion to take place outside of the brain-tissues after the optic vesicles or cups 

 had arisen. He says that Spemann did not at first advocate this late-fusion view, 

 but claimed (from his experiment on Triton) that the cyclopean eye arose out of 

 the medullary tissues in its final condition; subsequently, however, he assumed 

 the role of a most ardent supporter of the view that the fusion of the optic pri- 

 mordia takes place within the medullary plate. 



It may be added that there is no known instance of the formation of cyclopia 

 by experimental methods after the eyes are fairly well formed in normal develop- 

 ment. All the experiments in which cyclopia has been produced were made upon 

 the embryo at a stage before the eyes could be recognized under the microscope. 

 One must recall that Stockard's magnesium experiment is effective only when it 

 is done before the embryo is 15 hours old. In fact, Stockard found that the best 

 results were obtained if the eggs were placed in magnesium-chloride solution 

 immediately after fertilization. If the eggs are not placed in the solution until 

 15 hours after fertilization, before the germ-ring forms and begins its downward 

 growth from the yolk-mass, no cyclopia occurs. Cyclopia is less frequent in eggs 

 which are treated at later stages than in eggs immersed in magnesium-chloride 

 solution during the fourth and eighth cell stage. It appears, then, that the critical 

 stage at which cyclopia is best produced with magnesium is shortly before the 

 germ-ring is formed. According to Stockard, a 15-hour embryo has the germ-ring 

 beginning to form and descend over the yolk-sphere; the embryonic shield is 

 scarcely indicated, but appears soon afterward. Embryos of later stages sub- 

 jected to the same treatment develop normally, or at least do not show cyclopia, 

 while embryos younger than 15 hours, and at as early a stage as the first cleavage, 

 are much more readily affected in such a manner as to cause the cyclopean defect. 

 The optic vesicles appear at about 30 hours after fertilization, but the stimulus 

 must be applied at a time sufficiently long before this process occurs, since a num- 

 ber of important steps in eye formation are doubtless taking place before the 

 visible signs of optic vesicles are present. 



It is interesting to note that the Lewis pricking experiment is made at a stage 

 in which cyclopia can no longer be produced by placing the eggs in a magnesium 



