192 



E YCLESHYMER. [Vol. V 1 1 1 . 



Fi"". IV represents a section through a later stage when the 

 folds are still widely separated. There is but little change 

 in the histological character of the cells, except that their 

 boundaries are less distinct. 



Owing to division and loss of yolk, the cells of the embryo 

 have gradually become smaller and more compact. The 

 originally broad space enclosed between the neural folds has 

 undergone a constant reduction in size, a rapid closing-in of 

 the anterior portion has taken place, so that along the entire 

 length of the embryo the folds are approximated. A section 

 at this stage shows the condition represented in Fig. V. The 

 canal is elliptical owing to the slight evagination of the optic 

 areas which are the forerunners of the optic vesicles. Their 



Fig. IV. 

 Transverse section through optic pits. 



bilaterality is indicated only by the distribution of the pigment 

 and one might justly consider them as derived from a common' 

 anlage. A marked migration of the pigment has taken place: 

 instead of being located at the ends of the cells as in the earlier 

 stages, it is found between them and nearer the periphery. 

 The nuclei have likewise undergone a further migration toward 

 the surface, so that the cells of the superficial layer have com- 

 pletely lost their identity. 



It is of interest to note that in Petromyzon at a stage cor- 

 responding closely to the one above described, Kupffer* has 

 observed that an unpaired basis for the eyes is present, as the 



4 Arch. f. Mikr. Anat. 1890, p. 510. 



