OUTLTNES OF THE DEVELOPMENT OF THE TUATARA. 27 



sented on the ventral surface by two distinct dilatations (fig. 31), 

 while about the region of the mid-brain the lips of the medullary 

 groove are already united by a great proliferation of cells. 



In embryo 56 the medullary groove exhibits a sudden con- 

 tortion or curvature in the region of the hind brain (fig. 17), 

 but as this is more conspicuous in some later embryos I shall 

 postpone any further account of it, as well as of the closure of 

 the medullary groove, until I come to deal with them. 



Before leaving this stage it may be worth while to note 

 certain differences exhibited by the blastoderm in the neigh- 

 bourhood of the primitive streak, especially in embryo 64, as 

 compared with the corresponding parts at the preceding stage 

 (Z).), although these differences are probably due rather to 

 individual variation than to progressive development. The 

 differences in question are strikingly brought out by comparison 

 of figs. 14 and 34, representing transverse sections through 

 the primitive streak of embryos 58 and 64 respectively. In 

 the former, as already observed, there is a distinct ger- 

 minal wall, due to the sudden diminution in thickness of the 

 lower layer of the blastoderm at the juncture of the area opaca 

 with the area pellucida, so that only a very thin stratum of 

 lower-layer cells is continued across the area pellucida and 

 beneath the primitive streak. In the latter (fig. 34) the lower- 

 layer cells form a thick stratum both at the sides of and beneath 

 the primitive streak, though in the front part of the embryo 

 they thin out suddenly on meeting the margin of the area 

 pellucida as in embryo 58. Thus the primitive streak is sur- 

 rounded, except above, by a network of stellate cells with yolk- 

 spheres entangled in their meshes (compare alsofig. 35, Stage F). 

 In transverse sections of embryo 64, taken a little in front of 

 the primitive streak, the twofold origin of the mesoblast is 

 very clearly recognisable (fig. 33). The two lateral sheets of 

 mesoblast (P. S. M.), derived from the proliferation of the 

 compact rounded cells of the primitive streak, are seen thrust- 

 ing themselves in between the loose stellate cells of the original 

 lower layer {L. L, 31.), and dividing the latter into two parts, — 

 an upper very tliin layer which adheres to the epiblast, and a 



