18 



CYCLOPIA IN THE HUMAN EMBRYO. 



anterior end of the medullary plate. According to Lewis, certain groups of cells 

 in the medullary plate are predetermined to form the tapetum, the retina, and the 

 optic stem. Lewis's theory has been objected to by Bell, but it has been amply 

 confirmed by Spemann. At any rate, the arrangement of the structures in our 

 cyclopean embryo indicates that the optic stems have been cut out and that the 

 primordia of the retina and tapetum of the two sides have united, blending abso- 

 lutely with each other across the midventral line. 



The tissues of the midbrain and most of those of the interbrain appear to be 



normal, but our knowledge of 

 the normal brain at this period 

 of development is so scanty that 

 it is dangerous to make any defi- 

 nite statement. Single groups 

 of cells may be wanting or may 

 be blending without our noticing 

 the change. Such a blending 

 is clear only when it involves a 

 sharply circumscribed structure 

 like the eye. However, the 

 tissues of the hypothalamus 

 seem to be disarranged (plate 

 3, fig. 2), and those of the single 

 united cerebral vesicle (plate 3, 

 fig. 5) are certainly dissociated. 

 In the cerebral vesicle the cells 

 form a uniform layer, which is 

 not beautifully stratified, as is 

 the case in normal development. 

 Over the most anterior part of 

 the brain (plate 3, fig. 5), cross- 

 ing the midline, is a crescent- 

 shaped cap covering the outside 

 of the brain and reaching back 

 to the hypothalamus just below 

 the point of attachment of the 

 common optic stem (plate 3, 

 fig. 3) . This cap is composed of pale cells of uniform size, undoubtedly belonging 

 to the neural tube. It is located on the part of the brain which gives rise to the 

 olfactory lobes in normal development; and it may represent these lobes in a 

 degenerated form. 



It was necessary to make a model (enlarged 100 diameters) of the eye region 

 in order to study carefully the anatomy of the structures of the orbit. In this 

 model the nerves, from the third to the seventh, were worked out to their termina- 

 tions. The muscle masses, as far as they could be determined, are also included 

 in this model. 



5up Ob 



Sup. Ob. 



Fio. 1. Semi-diagrammatic section through the interbrain anil cyclopean 

 eye of embryo No. 559. X50. The cranial nerves are marked with 

 Roman numerals. Sup. Ob., superior oblique; EM., eye-muscle; M, 

 mouth. 



