CYCLOPIA IN THE HUMAN EMBRYO. 19 



The first branch of the fifth nerve is much thinner than the second and third, 

 and passes directly from the Gasserian ganglion back of the eye on either side of 

 the optic stalk; the branches anastomose with each other through several delicate 

 filaments back of the eyes, and then a larger bundle and several very small ones 

 enter the snout, to be lost there. The nerves are shown in section in the figures 

 on plate 3 and in reconstruction in plate 1 and figure 1. 



The fourth nerve takes its usual route and finally ends in a very large pre- 

 muscle mass lateral to the eye and to the first branch of the fifth nerve. It may be 

 noted that this arrangement of the fourth and first branches of the fifth appears 

 to be the reverse of the normal distribution according to Lewis's reconstruction 

 of our embryo No. 163 (fig. 368 of the Manual). In the Lewis reconstruction all 

 of the muscle of the primordium of the eye is blended into a single muscle mass, 

 while in the cyclopean embryo the premuscle mass of the superior oblique muscle 

 is entirely separated from the remaining premuscle mass of the orbit. 



The sixth nerve takes its usual course and ends independently in the hour- 

 glass-shaped premuscle mass which crosses the midline between the cyclopean 

 eye and the hindbrain (fig. 1). It is interesting to know that the transverse median 

 muscle mass, as well as the median anastomosis of the third nerve, occurs at the 

 point through which the pharynx gives rise to the hypophysis. In this embryo 

 the chorda ends in the pharynx behind this muscle mass and the pontine flexure 

 of the hindbrain. It appears as though, on account of the great amount of kink- 

 ing, the region of the infundibulum of the interbrain were pushed away from the 

 pharynx, thus making it impossible for the hypophysis to reach it. As a result 

 of this the third nerve and its muscle masses cross the midline. It may be 

 that the curious cytological changes in the muscle masses (plate 3, fig. 1, Oc.) 

 indicate that destructive changes are taking place in them, and that these small 

 round nuclei correspond with the Hofbauer cells as described by Essick in his 

 studies of the transitory cavities in the corpus striatum of the human embryo. 

 The primordium of the eye-muscles show some very remarkable cytological changes. 

 As the sixth nerve approaches the muscle mass of the lateral rectus it is at once 

 observed that this muscle falls into two sharply defined groups of cells, namely, 

 a median group which appears to be normal, and a lateral mass of smaller round 

 cells, the nuclei of which stain intensely. The same grouping is present in the 

 muscle mass of the third nerve. Near the midline the cells appear to be normal, 

 and laterally they are again composed of small round cells. The premuscle mass 

 at the end of the fourth nerve that is, the superior oblique muscle can be 

 outlined only with difficulty. 



The third nerve shows most remarkable changes in this specimen. It passes 

 along its usual course until it reaches the common eye-stem, over which it circles, 

 for the nerves from the two sides anastomose here, without any diminution in 

 size, within the common median primordium of the eye-muscle (fig. 1). This pro- 

 nounced anastomosis is also found in another cyclopean embryo, No. 201, in our 

 collection, as shown in figure 3. 



