large and the posterior significantly smaller. They were 

 clearly seen, because between them there were slits penetrat- 

 ing up to the pharynx; thus there was no doubt of their 

 existence. This information reminded me of the investigations 

 which I had carried out the previous winter on human embryos. 

 In the smallest of them, I did not discover branchial slits, 

 which were also absent in the embryos of other vertebrates, 

 although I frequently saw them in birds, frogs and snakes. 

 In those human embryos I investigated, I saw them more 

 clearly in that five weeks old embryo, than in the embryos 

 which I knew to be six weeks old" (p. 556). 



In the first specimen Baer saw three pairs of branchial 

 slits; the posterior pair was significantly shorter than the 

 others. The branchial slits were especially well seen from 

 inside the pharyngeal cavity. Based on comparison of avail- 

 able data Baer concluded that in humans and maybe in all 

 land vertebrates, there are primarily four pairs of branchial 

 slits, but they appear and disappear at separate times. In 

 his work illustrating the branchial slits of vertebrates, 

 Baer referred to investigations of Huschke,22 who discovered 

 in bird embryos that a vascular arch passes in every branchial 

 arch. This vascular arch begins from a general stem and comes 

 out from the heart; it then flows into the aorta which consists 

 of two roots. Thus every root of the aorta receives the 

 vascular arch of its side. Huschke, however, did not see all 

 the vascular branchial arches of the embryo. Baer himself 

 clearly saw in the three-week-old embryo of a dog four arches 

 filled with vascular blood, and he assumed that there was also 

 a fifth pair of very delicate arches not filled with blood. 

 He also distinguished four pairs of vascular arches in 

 chickens. 



Baer stated with certainty that the transformation of 

 the vascular system of mammals and birds is very similar 

 because the four pairs of vascular arches, which he saw in 

 the dog embryo, had great similarity to the four pairs of 

 arches of the bird embryos in the first half of the fourth 

 day after hatching. The comparison of the vascular system 

 of adult lizards and snakes, on one hand, and of embryos 



22. E. Huschke, "Uber die Kiemenbogen im Vogelembryo, " 

 ISIS (1828), pp. 160-164. 



436 



