No. 2.] EMBRYOLOGY OF THE LIZARD. 329 



as far as the eyes. Just anterior to their confluence in the ductus 

 cuvieri the cardinals give off on each side a vein that runs back- 

 ward immediately dorsal to the Wolffian organs, and supplies 

 each of these organs with a small vein running between the con- 

 volutions of the tubule. 



6. The Central Nervous System. 



The central nervous system in the earliest stage of my mate- 

 rial has been already referred to. Its chief features may be 

 seen in the transverse serial sections represented in Figs. 7, A, 

 and 18, A, inclusive. In general appearance it is a long tube, 

 anteriorly enlarged and curved ventrally. For a short distance 

 at both ends the tube is open dorsally, toward the middle the 

 walls meet dorsally, and in the middle region they are fused. 

 In the posterior open part of the tube the wall is a single layer 

 of columnar cells. Farther forward the lateral parts gradually 

 become much thicker, and the nuclei increase in number ; the 

 cells no longer extend from surface to surface of the wall, but 

 spindle-shaped cells appear internally. This condition obtains 

 throughout the anterior part of the tube. The floor of the tube, 

 however, remains permanently a single layer of cells. Owing 

 to the laterally compressed condition of the tube, this arrange- 

 ment is in some places difficult to observe, but it is well marked 

 in the floor of the brain. The floor may thus be distinguished 

 from the anterior medullary fold, which corresponds with the 

 lateral walls. 



At this stage may be seen the three anterior swellings of the 

 tube which correspond to the hind- mid- and fore-brain. The 

 latter is the part that is open dorsally. This opening extends 

 from the division of mid- and fore-brain to the dorsal edge 

 of the anterior medullary fold. In the lateral walls of this 

 primary fore-brain appear internally small depressions (C/, Fig. 

 9, A), which are the first indication of the optic outgrowths. • 



The lateral walls of the neural tube are largest in the head, 

 and taper gradually toward the posterior end of the body. In 

 the fore-brain the lateral walls appear to grow most rapidly, so 

 that by the time they fuse dorsally they enclose in diameter the 

 largest part of the neural canal. The anterior medullary fold 

 does not partake in this growth. Its growth ceases at an early 



