322 JOHN WARREN 



by a large irregular ingrowth from the mesial wall of the hemi- 

 sphere, its long axis extending vertically. The direction of the 

 hemisphere has also changed. In the previous figure it was 

 approximately horizontal, while here it is tipped distinctly for- 

 ward and downward. 



Fig. 12. This model is of Lacerta viridis of about 37 mm. We 

 notice at once a very great thickening in the brain walls and a 

 corresponding reduction in size of the cavities. The most strik- 

 ing feature is the change in shape and size of the dorsal part of the 

 diencephalon and in the relations between paraphysis, epiphysis 

 and pineal eye. The paraphysis has elongated, but its diameter 

 is much less than in the two previous stages and a number of 

 small tubules have appeared especially at the tip which touches 

 the tip of the epiphysis and has been thrust apparently in between 

 the epiphysis and the pineal eve, the latter now resting on the 

 upper third of the paraphysis. The change in relation between 

 paraphysis, epiphysis and pineal eye is a result of the approxi- 

 mation of the anterior and posterior parts of the post velar arch 

 following the continued shifting upward and forward for the mid 

 brain. This portion of the brain has altered its position still fur- 

 ther from that in the preceding stage. The caudal end of the mid 

 brain is now about on a plane with the velum, V., and a line 

 drawn between those points touches the lowest part of the pos- 

 terior commissure and is practically horizontal. If this is com- 

 pared with a similar line drawn from the caudal limit of the mid 

 brain to the posterior commissure in fig. 10 one gets a good idea 

 of the change in position of the mid brain. In fig. 10 such a line 

 forms an angle of about 80° with the horizon, while in fig. 12 

 the line is about horizontal. Therefore the mid brain has shifted 

 in a dorsal as well as in a cephalic direction, its walls at the same 

 time have become greatly thickened and its cavity proportionately 

 reduced. This forward development is indicated by the new 

 position of the superior commissure and anterior end of the pos- 

 terior commissure. These now lie directly over the optic com- 

 missure and have moved forward from the habenular flexure 

 to this point during the last three stages. This process has 

 approximated the anterior and posterior portions of the post 



