394 Harriet Lehmann, 



dog and showed the aortic arches in certain stages. In 1891, he also 

 figured aortic arches in the reconstruction of a human embryo twenty-six 

 days old. 



Zimmermann 1889 showed an interesting condition in a human 

 embryo of the fourth week, seven mm long, by demonstrating a recon- 

 struction of the same before the "Deutsche Anatomische Gresellschaft". 

 From the fourth aortic arch on each side, a vessel slightly smaller than 

 the arch itself, is given off between its first and second thirds. This 

 vessel is at first directed posteriorly, then takes a course parallel to the 

 arch, and unites with it again between the middle and last thirds. A small 

 epithelial evagination from the fourth entodermal pouch projects between 

 these two blood-vessels. Later, in the same year, Zimmermann described 

 a complete aortic arch between the systemic and pulmonary arches in a 

 rabbit of about the eleventh day of development. This rudimentary vessel 

 leaves the truncus arteriosus as a moderately slender vessel, broadens 

 gradually, until it becomes about half the diameter of the pulmonary arch, 

 and empties into the aorta close to the last aortic arch. It is separated 

 from the fourth and pulmonary arches by a clearly distinguishable ento- 

 dermal pocket. He also found in an incomplete series of sections of a 

 sheep embryo, a moderately large artery, leaving the distal end of the 

 pulmonary arch, and passing ventralwards midway between the fourth and 

 pulmonary arches, but was unable to trace its course fourther. From this 

 evidence he concludes that there are, in rabbit and human embryos, six 

 pairs of aortic arches, and five entodermal branchial pouches. He further 

 expresses the conviction that the same number of arches exist in other 

 mammals. It is to be regretted that Zimmermann's communications are 

 not illustrated. In 1890, he stated that the pulmonary artery in the 

 human embryo is not formed as Rathke has figured for mammals, but 

 that each pulmonary arch gives of a branch, as shown in the figures of 

 His and Boas. He also observed the subclavian artery to be given off 

 very near the bifurcation of the dorsal aorta, instead of near the arches 

 as Rathke has figured. However, since the subclavian in mammals, as 

 Hochstetter shows, changes its position, with relation to the dorsal 

 aorta and aortic roots during the process of development, it is probable 

 that neither Hathke nor Zimmermann describe the actual origin of this 

 vessel. 



Hochstetter (1890) made clear the origin of the subclavian arteries 

 in birds. He found that the primary arteries to the fore-limb arise from 

 the dorsal aorta. Shortly afterwards, secondary subclavian arteries develop 

 from the ventral end of the third arch, and unite in their course with 

 the primary arteries. The vessel arising from the dorsal aorta is at first 

 the larger, but as the ventral root of the subclavian increases in size, the 

 dorsal root degenerates and rapidly disappears. When this has been 

 accomplished the entire blood supply to the fore-limb comes from the 

 ventral end of the third arch. No diagrams, therefore, prior to HOCH- 

 STETTEr's paper, represent correctly the origin of the subclavian in birds. 

 In 1890, also, HOCHSTETTER published a figure showing a possible remnant 

 of a fifth aortic arch in the rabbit. This remnant was present in an 



