402 Harriet Lehmann, 



no actual connection can be traced to either bud. This tube-like 

 sinus is more distant from the ventral than the dorsal spur. A slight 

 ventral protuberance of the dorsal aorta, indicated at x in Fig. 4, 

 Plate 23, has the appearance of a bud in the sections of this region. 

 At a corresponding point on the right dorsal aorta a distinct small 

 bud is present. At y in the same figure is shown an isolated sinus. 



The first aortic arch in this embryo, from comparison with the 

 preceding and succeeding stages, may be considered to be at about 

 its maximum development as far as completeness is concerned. The 

 second arch is not perfectly symmetrical upon tlie two sides, nor 

 does it appear to correspond in its development to Tandler's de- 

 scription for the rat (p. 312), since upon the right side there is no 

 ventral spur, and the best developed part of the vessel is its middle 

 portion. In the right third arch a similar condition exists, except 

 that a ventral bud is also present. This indicates, that while buds 

 are usually formed from the dorsal aorta and the truncus, as Tandler 

 has described, it appears from my observations, that they do not 

 always grow toward each other and unite directly, but a middle 

 portion of the arch may also develop, and serve to connect the buds. 

 The evagination indicated at x in Fig. 4, has the appearance of a 

 dorsal bud of the fourth arch. The isolated sinus at y, I am unable 

 to explain. 



Embryo of the tenth day. Fig. 5, Plate 22, shows a sur- 

 face view of this embryo, which is in the tenth day of development. 

 The body flexure is well marked in the posterior end of the embryo, 

 and four gill arches are evident, instead of three as in the preceding 

 stage. 



Fig. 6, Plate 23, shows a reconstruction of the aortic arches of 

 the left side. Remnants of the first pair of arches still persist, and 

 complete second and third pairs are present. The latter run in the 

 hyoid and first branchial pouches respectively. The two dorsal aortae 

 are now united along a part of their course. The remnants of the 

 first pair are practically symmetrical on both sides, and consist of 

 a short ventral and dorsal remnant, with a series of discontinuous 

 sinuses lying between them. The second aortic arches are large 

 vessels with well established, unbroken walls, and show no signs of 

 degeneration. The third pair of arches are not symmetrical, the 

 arch of the left side being considerably larger than that of the right. 

 Although these vessels are complete, they are still much smaller 

 than those of the hyoid arches, and scarcely half the diameter of 



