PARIETAL EYE AND ADJACENT ORGANS IN 8PHEN0D0N. 115 



coagulum, in which at least one small nucleus is present 

 (fig. 4). A similar coagulum, with occasional nuclei, is visible 

 in the cavity of the fore-brain itself. 



Stage M. 

 At Stage M, comparable with a chick at about the middle 

 of the fourth day, with the limbs just beginning to bud out, 

 the primary parietal vesicle still remains as a simple outgrowth 

 opening into the cavity of the fore-brain, slightly to the left 

 of the middle line (fig. 5). 



Stage N. 



The Parietal Eye and Stalk. — At Stage N, comparable 

 with a chick at about the commencement of the fifth day, a 

 great advance has taken place in the development of the parts 

 under discussion. Longitudinal vertical sections of the fore- 

 brain at this stage (figs. 7, 8) show the parietal eye as a hollow 

 oval vesicle lying on the roof of the fore-brain, in front and 

 slightly to the left of its so-called '' stalk.-" The latter, which 

 I shall refer to in future as the "parietal stalk," is a glove- 

 finger-shaped diverticulum of the roof of the fore-brain, 

 curving forwards and lying nearly or quite in the middle line. 

 The parietal eye appears to be nearly if not entirely separated 

 from the stalk. It lies wedged in between the superficial 

 e|)iblast and the roof of the fore-brain, and is somewhat com- 

 pressed dorso-ventrally ; its front or dorsal wall is already 

 thickening to form the lens (fig. 8). The cavity of the stalk 

 communicates freely with the cavity of the fore-brain. In 

 histological structure its wall closely resembles that of the 

 parietal eye, being composed of columnar cells similar to those 

 of the wall of the brain of which it is an outgrowth. Both 

 stalk and eye contain the usual coagulum. 



It appears to me that there are two possible ways in which 

 the condition of the parietal eye and stalk at this stage may 

 have been derived from that which I have described as existing 

 at preceding stages. 



(1) The primary parietal vesicle may have divided into two 



