No. 2.] EMBRYOLOGY OF THE LIZARD. 319 



and 23, B, it will be seen that the ventral flexure of the brain 

 causes an angle to be formed between the ventral (or morpho- 

 logically anterior) surface of the head and the external surface 

 of the oral fusion and lower jaws. The apex of this angle of 

 surfaces is seen in the line of epiblast, Ed and Hph, in Fig. 22, 

 B. In 23, B, the space i^M.iii) enclosed by the angle is seen. 

 The apex of the angle, therefore, extends from side to side of 

 the head in nearly a straight line. As the brain and lower jaw 

 increase in size, the angle becomes more and more acute. In 

 Fig. 48, D, this angle (i/) is shown in a lateral, longitudinally 

 vertical section. In the median line, posterior to the apex of 

 the angle, the mouth-opening breaks through. The lower jaw, 

 thus released in the middle, curves outward, and, overlapping the 

 epiblast anterior to the apex of the angle, it begins its growth 

 forward, which eventually brings its median part to the nasal tip 

 of the head. 



Between the apex of the angle referred to and the dorsal 

 edge of the mouth-opening there remains a small portion of 

 the posterior epiblastic surface of the angle. This small portion 

 of epiblast forms the posterior wall of the hypophysis; it is 

 visible in Fig. 49, D, under the end of the notochord and pos- 

 terior to the space J/, and also in 50, D, — a slightly lateral 

 section (^Hph). Owing to the lateral twist of the head of the 

 embryo, it is impossible to make a truly median, longitudinally 

 vertical section ; although section 49, D, passes through the 

 median plane at the notochord, it cuts also the lateral boundary 

 of the mouth-opening just behind the space J/ (Fig. 49, D). 

 (I have omitted to reproduce this posterior wall of the hypo- 

 physis in the reconstructed section of Fig. 51, D.) As soon as 

 the hypoblast separates from, and sinks down from the noto- 

 chord, the characteristic appearance of the hypophysis becomes 

 much more pronounced, and appears as pictured in Fig. 47. 

 Here the hypoblast blends indistinguishably with the posterior 

 wall of the hypophysis. The cavity of the hypophysis has 

 meanwhile become enclosed laterally by the tissue at the sides 

 of the hypoblastic mouth {AI, Fig. 22, B) growing forward to 

 form the upper jaw. The expansion of the head-cavities may 

 possibly help to cause this effect. The later condition of the 

 hypophysis may be seen in Figs. 33, C, and 34, C. Here the 

 brain is increased very much in size, and a ventral distension of 



