OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 29 



and a few mesoblast cells make their way in between the two 

 (fig. 45). It is very narrow, and the cells of its roof are much 

 more clearly differentiated from the surrounding mesoblast than 

 those of its floor. 



In both embryos of this stage the medullary groove has 

 become closed in the region of the mid-brain by the union of 

 its two very prominent lips to form a thick mass of cells, 

 which projects backwards so as to overlap the hind brain. 

 The thickening of the lips of the medullary groove in this 

 region is already prominent at the preceding stage, as shown 

 in fig. 27. Fig. 42 shows the thickened lips united to form 

 the roof of the mid-brain {R. M.). Fig. 43 shows this roof 

 overlapping the hind brain, and fig. 44 is through the hind 

 brain a little further back, showing the medullary groove still 

 widely open. Fig. 35, representing somewhat diagrammatic- 

 ally a median longitudinal section of embryo 61, shows that it 

 is only in the region of the mid-brain that the closure of the 

 medullary groove has become completed. 



In both these embryos also, but especially conspicuous in 

 No. 72 (fig. 39), tliere is a sharp bend in the region of the 

 hind brain. This bend appears to be due to the restraining 

 influence of the pro-amnion, which attaches the head to the 

 under surface of the blastoderm in the region of the mid- brain, 

 and seems to prevent the embryo from elongating in a straight 

 line. The result is that the head appears to be pulled up- 

 wards and backwards, as a horse's head is pulled back by the 

 reins (compare fig. 36, and Stage G, fig. 50). This pulling 

 back of the head probably accounts for the overlapping of the 

 hind brain by the roof of the mid-brain (compare figs. 39 and 

 43, R. M.). 



In embryo 72 the neurenteric canal is still closed below, but 

 in No. 61 it opens widely into tlie future alimentary canal 

 just behind the notochord, and in front of the great mass of 

 the primitive streak. This opening is clearly shown in the 

 longitudinal section represented in fig. 35, which also shows 

 very clearly how the notochord appears as a forward growth 

 from the region of the primitive streak, or, to speak more 



