CYCLOPIA IN THE HUMAN EMBRYO. 23 



It is somewhat easy to compare the medullary plate of the Bartelmez embryo 

 No. 1201 with the medullary plate in amphibia. The lateral foveola corresponds 

 with Eycleshymer's pigmented spots, and if these areas were cut out eyeless embryos 

 should be produced, as Stockard found in his experiments on Amblystoma. If it 

 is admitted that the eye primordia communicate across the midline through the 

 torus opticus, then Stockard's experiments upon the medullary plate may be 

 interpreted as follows: 



No important organ develops from the midline of the medullary plate, and 

 this is represented only by a thin layer of cells. It has been called by His the 

 floor-plate. The motor nuclei arise from the thickenings on either side of the 

 floor-plate, and these are known as the basal plates. The narrow, thin floor- 

 plate really forms the ventral midseptum of the spinal cord of the brain, and sub- 

 sequently commissural fibers grow through it to form the raphe. If we view the 

 basal plate from above, as shown in embryo No. 1201 (plate 2, fig. 6), we find that 

 this raphe should extend forward to the neuropore, at which point the raphe 

 fibers are the anterior commissure within the torus transversus. Back of this we 

 have the torus opticus, and its commissural fibers are the fibers of the optic nerve. 

 At an early period in its development the torus opticus must widen rapidly and 

 push through the rest of the brain, for the optic stalk appears quite suddenly, 

 and an injury to the medullary plate at this time would probably make itself felt 

 more upon the optic stalk than upon the eye, for the short period in which it grows 

 very rapidly is its critical period. If we cut out the optic stalk or the torus opticus, 

 as Stockard and Lewis did in their experiments, then the foveola would remain 

 together and form cyclopia. Stockard and Lewis also note that in their experi- 

 ments they frequently obtained embryos with but one eye which appeared to be 

 quite normal. This is to be expected when experiments are made upon two prim- 

 ordia which lie very close together; and when either the left or right eye primor- 

 dium is removed, left or right-eyed embryos are produced, but not cyclopia. 

 Lewis had to destroy only the midline of the embryonic shield in order to produce 

 true cyclopia. It may be added that the anatomical changes found in our small 

 cyclopean human embryo, as well as in all cyclopean human monsters, can be 

 explained by removal of the structures represented along the line of the raphe 

 of the medullary plate reaching from the mammillary bodies to the neuropore. 

 This includes the torus transversus, which naturally involves the olfactory region 

 and the anterior commissure. Thus we can explain by a study of this specimen 

 the anatomical changes of the brain found in human cyclopia. 



STUNTED EMBRYO, GL 20, WITH CYCLOPIA, CARNEGIE COLLECTION, No. 201. 



The second specimen of cyclopia in our collection is a pathological embryo, 

 which is not very well preserved, measuring GL 20 mm. This specimen came 

 from Dr. Schelly of Baltimore, who obtained it from an abortion on February 7, 

 1902. He brought it to Mr. Brodel, and subsequently, because it was patholog- 

 ical, it was given to me. The embryo was incased in an ovum covered with a 

 ragged decidua, together measuring 80 by 60 by 50 mm. Upon opening this 



