412 Habriet Lehmann. 



Embr3^o of the twentieth day (No. 2). This embryo is 

 estimated to be about the same age as the preceding one, but it 

 shows certain variations in the condition of the aortic arches worthy 

 to be noted. 



In external view (Fig. 15, Plate 22) there are three well marked 

 branchial pouches, and a fourth similar to that in the preceding 

 stage. Judged from the appearance of the mandibular pouch and 

 the nasal epithelium, this embryo is slightly older than that shown 

 in Fig. 13, Plate 22. 



The internal conditions as represented in Fig. 16, Plate 24, show 

 three complete arches, the third, fourth and sixth, and the first and 

 second arches are also nearly complete. The rudiment of the first 

 arch consists of a long, slender vessel passing downward from the 

 dorsal aorta through the mandibular visceral arch. It extends to 

 the level of the ventral end of the second arch, where it bends 

 toward the interior, and ends blindly, only a short distance from a 

 forward prolongation of the truucus arteriosus. This remnant of 

 the first aortic arch, unlike the corresponding one in Fig. 14, Plate 23, 

 is fairly regular in its course, and few sinuses lie about it. 



The rudiments of the second arch are large, and in reconstruction 

 appear to overlap each other, but this appearance is misleading for 

 they lie at different planes, and in the reconstruction are projected 

 one against the other. Near its ventral end, the single channel of 

 the dorsal rudiment breaks up into several, which end blindly. A 

 few disconnected sinuses also exist in the region of the truncus 

 arteriosus. The latter projects forward aud upward beyond the 

 ventral termination of the third arch, in the form of a large blind 

 pocket, and two slender branches directed both laterally and forwai'd 

 from its anterior wall, appear to be remnants of the ventral ends 

 of the second pair of arches. The more posterior ends of these 

 branches are but a short distance from each other. 



The third arch is not relatively as large as in the preceding 

 stage, but the fourth has increased in size making these two vessels 

 nearly equal in diameter. The clear space at the dorsal end of the 

 fourth arch marks an infolding of the wall which passes through 

 the vessel. It is the same structure noted in the previous specimen 

 and corresponds to the "Inseln" of Tandlek, 



The sixth arch is complete, and shows an enlargement at its 

 dorsal end, which joins the aorta upon the inner border of the 

 latter vessel. Each pulmonary arch gives rise about midway in its 



