12 CYCLOPIA IN THE HUMAN EMBRYO. 



solution. According to Lewis, the experiment should be made on the second 

 day. Although he does not give his experiment in hours, his illustrations show 

 the stage of development. According to these the embryonic shield is well formed. 

 The experiments of Lewis were first described in my monograph on monsters, 

 but they have since been reported in detail by their author. As has been stated, 

 Lewis produced cyclopia in- Fundulus by pricking the middle of the anterior end 

 of the embryonic shield two days after fertilization. In the course of a few days 

 it became apparent that in some of the eggs operated upon the eyes had developed 

 normally, while in others they had become cyclopean. Most of the specimens 

 were killed after 15 days. Pricking of the embryonic shield was accompanied 

 by the escape of a slight amount of tissue, and as there is little or no regeneration 

 of the central nervous system in Fundulus, the defect caused at the time of pricking 

 subsequently became more and more apparent as development proceeded. Both 

 Lewis and Stockard have found that cyclopean Fundulus embryos usually develop 

 with a normal brain, thus no doubt accounting for the vitality of this special 

 cyclops. Furthermore, it appears that the eye primordium in Fundulus is more 

 circumscribed than in many other animals. 



In a number of his experiments Lewis found that the material withdrawn 

 with the needle-point came from one side of the anterior end of the embryonic 

 shield, with a resulting abnormality of the eye on that side. In a specific case, 

 at the time of hatching, the right eye of the specimen consisted of a small bit of 

 retina connecting with the otherwise almost normal brain-wall. The left eye was 

 apparently normal, as were also the brain and the nasal pit. In other specimens, 

 in which the operation was about medial and was done at the time the embryonic 

 shield was beginning to form, the embryos developed with the two eyes in contact, 

 with two optic nerves and two lenses. Among other specimens there is one with 

 a cyclopean eye, having a layer of pigment narrowing between the two eyeballs. 

 In specimens operated upon at a little later stage there is a median cyclopean eye 

 with two lenses, one pupil, and one cup cavity. Using Lewis's language, the 

 large optic cup shows in sections a very beautiful median eye with complete con- 

 tinuity of the layers of the retina of two components about a single large cup 

 cavity of a single lens. 



According to Lewis, the explanation of these various abnormalities is in a way 

 comparatively simple, if we assume that in the early embryonic-shield stage the 

 various parts of the central nervous system and the eyes are probably already 

 predetermined, and, secondly, that there is very little or no power of regeneration 

 in this tissue. Numerous experiments on regeneration indicate very clearly that 

 there is little or no regeneration of the tissue (at least of that of the central nervous 

 system) extruded at the time of the operation. The repair which takes place 

 after the operation consists merely of a rapid closing together of the parts left 

 remaining, and thus a healing of the wound occurs without regeneration of lost 

 parts. This closing of the wound is accomplished in a few minutes, and primordia 

 are thus brought into contact which normally are quite widely separated those 

 of the two eyes, for example. The subsequent differentiation adjusts itself to the 



