GILL-FILAMENTS IN SAUROPSIDA 



211 



The pectoral grooves first appear about the middle of the sixth 

 day and at once deUmit a roughly triangular area whose base 

 coincides with the intersection of the neck and breast, and whose 

 downward directed apex lies just above the umbilicus. At first 

 the area enclosed by these grooves is transparent throughout, 

 revealing the outlines of the heart beneath. But almost immedi- 

 ately the basal third becomes vascular and much thicker than 



Fig. 1 Sketch of the markings on the pectoral wall of a seven-day chick (H. 

 E. C, Ser. 2076; 17.3 mm.; 6 days, 7 hours) together with three transverse sec- 

 tions of the pectoral wall of the same embryo, X 42 diam. 507, section at level 

 of cp.; 550, section at level of s.p.; 651, section at level of c.c, m.L, andm.p., 

 lateral and pectoral margins of opercular fold; i.o., opercular notch; t.o., opercu- 

 lar tubercle; f.hr., row of branchial filaments; a.o., opaque area; c.p., pectoral 

 ridge; s.p., pectoral groove; c.e., epitrichial ridge; cm., mesothelial ridge; u. 

 umbilicus. Compare with frontal section of pectoral wall in figure 20. 



the apical portion which retains for some time its non-vascular 

 and transparent character. In fresh specimens the upper portion 

 exhibits a semi-opacity somewhat similar to that of ground glass. 

 It is this part which at the beginning of the seventh day gives 

 rise to a series of superficial evaginations which may appear 

 anywhere in this area, but are chiefly ranged along the medial 

 borders of the pectoral grooves. Ultimately those of a side be- 

 come numerous enough to form a pair of pectoral ridges which 

 in their later development, as previously noted, exhibit a super- 



