No. 2.] EMBRYOLOGY OF THE LIZARD. 327 



mental duct. Its size increases anteriorly, until in the body 

 segments behind the heart-region it unites with the most ante- 

 rior, and accordingly the oldest, Wolffian bodies, as above 

 described. Later stages show that each Wolffian body becomes 

 a much convoluted tubule, with the median end of its lumen 

 separated by a thin membrane from a protruding pocket of the 

 aorta, while distally its lumen connects with that of the seg- 

 mental duct, which opens into the cloaca.^ (See series B.) 



In stages about the time the Q^'g is laid may be seen the 

 method of development of the mesoblast of the tail. In trans- 

 verse sections through the anterior region of the large neu- 

 renteric canal, and just in front of it, the intestine presents a 

 vertically oval lumen, with a wall of distinct columnar cells. In 

 the dorso-lateral parts of the wall the cells are much elongated, 

 and extend their free spindle-shaped ends outward. A band of 

 similarly shaped cells on each side connects this part of the 

 wall of the intestine with the corner of a triangular solid mass of 

 mesoblast, which lies dorsal to the intestine between the neural 

 tube and the epiblast. Aside from the cells just mentioned 

 no others appear between the hypoblast and epiblast. A few 

 sections farther forward this cell-proUferation ceases, and the 

 mesoblast unites ventrally. 



This derivation of the caudal mesoblast corresponds to a very 

 general method of origin of the mesoblast, from the dorso- 

 lateral walls of the alimentary tract. This appears also to be 

 the method of origin of the head-cavities. 



5. TJie Circulatory System. 



The circulatory system of the lizard in these early stages 

 presents but few peculiarities of difference from the accounts 

 given of other lower vertebrates. In general it agrees with 

 the description given by Shipley of the same system in Pe- 

 tromyzon. 



In the space between the oral fusion and the opening of the 

 head-intestine into the yolk-sack the splanchnopleures fold in- 



' It was not at first my intention to treat of the primitive kidney in this paper. 

 When I became impressed with the significance of what I described above it was 

 too late to add new figures to ray plates, which had already been sent to the litlio- 

 grapher. I hope to publish, at another time, figures illustrating what I have here 

 described. 



