CYf'LOPIA IN THE HUMAN EMBRYO. 27 



branch of the fifth nerve runs deep into the neck and is separated from the second 

 by a pronounced ossification center representing, of course, the maxillary bone. 



The two eyes are united, forming an hourglass-shaped body with a double 

 retina, a single pigmented layer, and a single optic nerve which arises from the 

 retinas as they approach each other (figs. 4 and 5). The tapetum is continuous 

 over the superior surface of the eyes, but it is broken below, repeating the 

 condition found in the eye of embryo No. 559. The optic nerve reaches to the 

 base of the interbrain, where it ends abruptly. It is impossible to determine 

 with precision the arrangement of the muscle masses of the orbit or of the nerves 

 passing to them. That no trace of the sixth nerve could be made out is shown 

 in figure 3, and this may be accounted for by the fact that the organ which gives 

 rise to the sixth nerve has undergone extensive degeneration. However, the 

 peripheral ends of the third and foyrth nerves can be found, but they can not be 

 traced back very far in the direction of their origin. The fourth nerve is thicker 

 than normal and ends on the lateral side in an enlargement which may represent 

 the superior oblique muscle. Below the optic nerve is a common muscle mass 

 which crosses the midline, and to either side of this there are two independent 

 muscle masses. Before the third nerve reaches these muscle masses lateral 

 branches are given off, which pass to the second lateral muscle mass, as shown in 

 the diagram. The two nerves then approach each other and communicate freely 

 through the unpaired muscle mass, and then pass forward under the cyclopean eye 

 and finally end just beneath the skin. 



A comparison of plate 1, figure 1, and text-figure 1 with text-figure 3 shows 

 that the first branch of the fifth nerve in both embryos anastomoses across the 

 midventral line in the region of the snout and that the two third nerves anastomose 

 with each other through the main muscle primordium of the orbit. Both of these 

 anastomoses must be viewed as secondary, for the two nerves must have been single 

 when they arose from the brain. This observation favors the theory that the eye 

 promordia must also have been bilateral that is, they must have been separated 

 by a narrow strip of non-ocular tissue in the normal medullary plate. 



FETUS COMPRESSUS WITH CYCLOPIA, CAHNEGIE COLLECTION No. 1165. 



This embryo was sent to me by Dr. Ralph S. Perkins, of Exeter, New Hamp- 

 shire. Only the embryo was received, which measures 43 mm. CR. It was 

 found to be greatly distorted; the umbilical cord is of thread-like thinness, and 

 the development of the different parts of the body seems to be unequal. Appar- 

 ently some of the joints are dislocated, but at present it is impossible to say 

 whether or not these distortions are due to mechanical manipulation after the 

 embryo was aborted. This is possible, because the embryo had been wrapped in a 

 towel some time before it was fixed in formalin, but a careful study of the sections 

 demonstrates that the specimen is quite a typical fetus compressus. 



The embryo came from a white woman, 35 years old. Her first child is 16 

 years of age; the second died at the age of 2, and the third is 12 years old. 

 These are all by her first marriage. The first pregnancy of her second marriage 



