26 



CYCLOPIA IN THE HUMAN EMBRYO. 



I'm. 4. Section through the cyclopean eye of embryo No. 201. X40. 

 1", first branch of fifth nerve; S, snout. 



isolated brain-mass between the eye and the large mass just described. By com- 

 paring these structures with a profile reconstruction of the forebrain of the cyclo- 

 pean embryo No. 559 (plate 1, fig. 1), the larger mass just above the ganglion of 

 the fifth undoubtedly represents the interbrain, as the free end of the optic nerve 



reaches just to its base but does 

 not enter it. The crescent- 

 shaped mass of the brain in 

 front of the interbrain is the 

 unpaired single cerebral vesicle, 

 which communicates freely with 

 the ventricle of the interbrain. 

 The structure of the wall of this 

 degenerated cerebral vesicle cor- 

 responds very closely with that 

 of the small vesicle in embryo 

 No. 559. With these structures 

 established, as shown in the 

 profile reconstruction, it is pos- 

 sible to identify some of the re- 

 maining structures of the orbit. However, the tissues are very greatly dissociated, 

 and for the present it is impossible for me to follow them farther than is given in the 

 following description. First 

 of all, the three branches of 

 the fifth nerve can be fol- 

 lowed to their termination. 

 The first branch reaches up 

 to the rudimentary snout, 

 where it anastomoses with 

 its fellow from the opposite 

 side shown also in the dia- 

 gram (fig. 3). This snout, 

 by the way, is represented 

 by a slight elevation in the 

 middle of the face above the 

 eye, and sections through it 

 show that it is composed of 

 a relatively large, irregular 

 mass of cells which are sepa- 

 rated more or less from the 

 surrounding mesenchyme. Within the middle of this mass is a small pearl-like 

 bod}' about 0.1 mm. in diameter. Just behind this body the first branch of the 

 fifth nerve anastomoses across the midline. 



The second branch of the fifth runs below the eye on either side and reaches 

 nearly to the skin, where it spreads out into several large branches. The third 



FIG. 5. Section through the cyclopean eye and optic nerve of 

 embryo No. 201. X40. 



