OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 77 



The Wolffian Ducts and Tubules. — These organs first 

 appear at Stage J. Their development has not been followed 

 in any detail, but appears to take place very much as described 

 by Weldon (6) in Lacerta (vide figs. 68, 69, 80). 



The Vascular System. — The heart was first recognised 

 at Stage H (fig. 52) as a somewhat pear-shaped sac, lying in 

 the pericardium, receiving the two vitelline veins behind, and 

 giving off the short bulbus arteriosus in front. It probably 

 originates as a split in the splanchnopleuric mesoblast. At 

 Stage J, when it was first observed in sections, it looks like a 

 ventral diverticulum of the splanchnopleuric mesoblast, here 

 composed of short columnar cells, containing two thin-walled 

 epithelioid tubules (fig. 66), continuous with the vitelline veins 

 behind, and uniting in front to form the bulbus arteriosus, 

 while the whole heart has begun to be bent into the form of 

 an S (fig. 59). A full description of the circulation at Stage J 

 has already been given, and need not be repeated ; it shows no 

 features of special interest, except perhaps the enormous dila- 

 tation of the primitive aortse on either side of the anterior 

 extremity of the alimentary canal (fig. 62). Beyond Stage J 

 the development of the vascular system within the body has 

 not been worked out. The development of the vitelline 

 vessels has already been referred to in dealing with the yolk- 

 sac. 



The Teeth. — The only feature of special interest observed 

 in connection with the teeth concerns the development of the 

 two large cutting teeth which are so conspicuous in the front 

 part of each jaw of the adult. At Stage S, shortly before 

 hatching, each of these is represented by three distinct, pointed, 

 conical teeth. No vomerine teeth were observed. 



{d) The Embryonic Colour Markings. 

 One of the most remarkable features of the development is 

 the appearance at Stage R of a well-defined embryonic pattern 

 on the integument totally different from that of the adult. 

 This pattern consists mainly of two distinct series of markings : 

 (1) a series of narrow, discontinuous, longitudinal stripes of 



