No. 2.] EMBRYOLOGY OF THE LIZARD. 323 



On the ventral surface of this large gill chamber, inter- 

 mediate to the regions of the first and second clefts, appears the 

 first rudiment of the thyroid gland (Fig. 31, C). In horizontal 

 section its outline is a circle. It is a compact thickening of 

 the wall of the gill chamber, and its cells lie in a radiate posi- 

 tion. Immediately ventral to it is the fork of the ventral 

 aorta. In a later stage this round thickening of cells has be- 

 come depressed in the centre, so that it has a lumen slightly 

 constricted at its opening to the gill chamber. Later it appears 

 entirely separated from the wall of the alimentary canal, and 

 without lumen. Finally it assumes its usual shape and position, 

 with its thin median part lying ventrally across the trachea, a 

 little in front of the separating bronchial tubes. 



After the breaking through of the primitive mouth, as already 

 stated, the lower jaw, overlapping the primitive anterior surface 

 (anatomical base) of the brain, grows toward the nasal tip of 

 the head. The progress of this growth maybe seen in Fig. 51, 

 D, where the jaw is a little in advance of the hypophysis ; and 

 Fig. 63, F, PL XVI., where the jaw extends beyond the optic 

 chiasma. 



The lumen of the gill chamber gradually decreases pos- 

 teriorly until, just behind the region of the fifth cleft-rudi- 

 ment, the ventral half of the lumen seems to be obstructed 

 by a transverse wall. Dorsally the lumen continues, but so 

 small that a single one of its limiting cells would suffice to fill 

 it. From this transverse, obstructing wall (Zr, Fig. 36, C) is 

 developed the lung rudiment. First appears a small tube 

 growing out posteriorly and parallel with the intestine. This 

 tube soon divides into two similar tubes, which continue their 

 growth backward, though separating laterally. All these tubes 

 are provided with a lumen, and show the same columnar 

 endothelium as the intestine. A distal expansion of these 

 lateral tubes finally leads to the growth of two large bladder- 

 like sacks. At one period, when these sacks occupy relatively 

 the entire space intended for the lungs, they possess simple 

 large oval cavities without any reticulation. They have, how- 

 ever, small ridges on their internal surface through which run 

 blood-vessels. These ridges soon increase in size, and, extend- 

 ing into the cavity, cause the reticulation. 



Behind the origin of the trachea, and opposite the posterior 



