44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



cup (oc), ill whose opening lies the lens vesicle (Iv), now reduced 

 to a crescentic slit by the thickening of its posterior wall. The 

 mesoblast is more dense in those parts of the section adjacent to the 

 pharynx than in the more distant regions, and the ectoderm thickens 

 in a marked way as it approaches the borders of the pharynx and 

 gill clefts. Only a few small blood-vessels (bv) are to be seen in 

 the region of the forebrain. 



Parts of three pairs of clefts (g) are shown in the figure: one 

 pair opens widely on either side, so that there is a large area of the 

 section that is distinct from the two still larger portions and con- 

 tains a small, thick- walled cavity (g) on the right side; this cavity 

 is a gill cleft that is cut through neither its outer nor its pharyngeal 

 opening. 



No structures other than this small portion of a gill cleft and a 

 few blood-vessels are to be seen in this middle region of the section. 

 In the more posterior part of the section, in which the notochord 

 (nt) is located, a pair of curved clefts may be seen, opening entirely 

 through the wall on the left, but closed on the right (g). One dis- 

 tinct pair of aortic arches is shown (ar) , and also the dorsal aortae 

 (ao), which are of very unequal size. The spinal cord (sc) and 

 muscle plates need no special description. 



Figure i6d is in the region of the heart (ht) and lungs (lu). The 

 former is an irregular cavity whose walls, especially on the ventral 

 side (mes r ) are becoming very thick and much folded. Although 

 thin, the body wall completely surrounds the heart, as would be 

 expected, since this was true of the preceding stage. The lung 

 rudiments (lu) and the foregut from which they have arisen have 

 the same appearance as in the chick ; they consist of three small, 

 thick-walled tubes so arranged as to form a nearly equilateral tri- 

 angle. They are surrounded by a swollen, rounded mass of meso- 

 blast which almost completely fills the surrounding portion of the 

 body cavity (be). The pleural sides of these crescentic portions of 

 the body (or pleural) cavity — that is, the boundary of the mass of 

 mesoblast just mentioned — is lined with a thickened layer of cells, 

 shown by the solid black lines in the figure. The lung rudiments 

 may be traced through about fifty sections of this series, or about 

 one-twelfth of the entire series. At the dorsal angle of the part of 

 the body cavity (be) just described, near the dorsal aorta (ao), are 

 two dark, granular masses (ge), which, under a higher magnifica- 

 tion than is here used, are seen to consist of a small group of blood- 

 vessels filled with corpuscles; although several sections in front of 

 the anterior limits of the kidneys, these are evidently glomeruli. 

 They may be traced, though diminishing in size, far toward the 



