No. 2.] EMBRYOLOGY OF THE LIZARD. 331 



of the eyes. This region would correspond in a much older 

 embryo (Fig. 51, D), as nearly as can be estimated, to the space 

 between the constriction of the brain, c, and the point og, which 

 lies directly between the two hollow optic stalks. In embryos a 

 little older than that of Fig. 45, but much younger than that of 

 51, D, there is a linear median fusion of the dorsal and part of 

 the anterior wall of the fore-brain with the cpiblast. Every- 

 where else the brain is separated from the epiblast. The latter 

 consists of a single layer of cubical cells. This fusion extends 

 exactly through the region of the slit above described, and in- 

 dicates the line of the closure of the fore-brain. Along the 

 middle region of the fusion is a linear external depression, — 

 the last remnant of the slit. The fusion is perhaps more com- 

 plete at its ends, but its general appearance has led me to the 

 conclusion that the entire slit closed nearly simultaneously. 



In view of what I have found to be the case in the embryos 

 examined by me, I am at a loss to account for the statements of 

 Hoffman in this respect. He has figured a median longitudinal, 

 vertical section of the entire head of an embryo of Lacerta, in 

 which in the middle dorsal region of the primary fore-brain 

 there is a pore-like external opening of the ventricle, the ad- 

 jacent brain-wall being continuous with the epiblast. This, he 

 says, is the last point of the enclosure of the brain. The lateral 

 twist of the head would make it impossible to obtain any such 

 complete median vertical section in all my embryos of this stage. 

 In my youngest specimen the morphologically vertical plane 

 of the head is at an angle of about 45° to the vertical plane 

 of the trunk. In later stages, sections parallel to the vertical, 

 longitudinal plane of the trunk and hind-brain, but slightly to 

 one side, would cut the slit {0) in the fore-brain obliquely, 

 so that it would present in each section the appearance of 

 a pore. 



Shortly after the complete closure of the brain and its separa- 

 tion from the epiblast, its vertical median section appears as 

 represented in the reconstructed section of Fig. 51, D. The 

 epiblast is a thin membrane of flattened cells, and lies close to 

 the roof of the brain. The roof of the hind-brain is also a thin 

 membrane. There is no dorsal constriction between the hind- 

 brain and mid-brain, but they are separated by a marked lateral 

 constriction. The mid- and fore-brain are separated by an encir- 



