Embryonic history of the aortic arches in mammals. 417 



pulmonary arteries is clearly according' to the manner described by 

 Bremer. 



Embryo near the close of the twentieth day (No. 5). 

 This embryo is slightly older than the preceding-, but from com- 

 parison with the figures of Keibel still belongs to the twentieth 

 day of development. 



The external view (Fig. 21, Plate 22) shows four branchial 

 pouches. The mandibular has a longer maxillary process than in the 

 preceding- stage, and the third bar shows a horizontal groove a short 

 distance above its middle. 



In the reconstruction (Fig. 22, Plate 24), are shown small dorsal 

 rudiments of the first and second arches, of about equal size. 

 A chain of disconnected sinuses lies in the mandibular and hyoid 

 arches in a position similar to the large sinus shown at a in the 

 preceding reconstruction (Fig 20, Plate 24). These have not been 

 shown in the reconstruction on account of the difficulty in representing 

 clearly such irregular structures. These sinuses are so closely asso- 

 ciated toward the ventral ends of the third arches, that the}^ seem 

 to outline the course of an incomplete vessel, but no clear connection 

 can be determined between these sinuses and two small branches 

 from the ventral ends of the third arches. They end, however, close 

 to each other. The vessels just mentioned (Fig. 22, Plate 24) leave 

 the third arches a short distance above their union. That of the 

 left side is somewhat nearer the median plane than the right. The 

 noticable pocket-like extension of the truncus arteriosus present in 

 younger pig embryos was not found in this stage. 



In connection with the fourth arch are structures similar to 

 certain of those described for the preceding embryo as rudiments of 

 a fifth arch. Near its ventral end, a short spur leaves the fourth 

 arch, directed toward the dorsal end of the sixth. In the posterior 

 side of the fourth arch, near its union with the dorsal aorta, is a 

 small ingrowth of the surrounding tissue. Below this structure, a 

 slender vessel leaves the dorsal aorta and runs close to the latter 

 vessel, joining the sixth arch at its dorsal end. This vessel takes 

 a course similar to the posterior portion of the fifth arch in the 

 following stage. Its walls are not as regular, and trabeculae appear 

 in its channel, but it is nevertheless a complete vessel. Two ento- 

 dermal branchial pouches are present between the fourth and sixth 

 arches, but the more dorsal one is not so well developed as in the 

 preceding stage. 



