Embryonic history of the aortic arches in mammals. 399 



the formation of the aortic arches, while the pig- series carries the 

 story further along the line of transformation. Many variations occur 

 in the aortic arches in individual embryos, and where the two forms 

 can be compared, the history of the arches while similar, is not 

 identical. 



In order to fix the stages in which particular phases of develop- 

 ment occur, careful external drawings were made of each embryo 

 studied. No external drawings of the rabbit were available for 

 reference, and, Keibel's excellent figures of swine embryos, correspond 

 in detail in only few instances, to the pig embryos which were 

 finally sectioned and reconstructed. The changes in the aortic arches 

 take place in such a closely graded series that it would be impossible 

 to find a correspondence in each case with the figures of another 

 author. 



The figures illustrating the aortic arches in the pig and rabbit 

 are from carefully measnred reconstructions. To obtain the latter, 

 a camera sketch was made of each embryo to be sectioned; the 

 plane of the sections (as nearly as possible at right angles to the 

 general direction of the aortic arches), was accurately determined 

 and they were cut of uniform thickness; each successive section in 

 the region of the aortic arches was drawn with the aid of the 

 camera; the outline of the external drawing of the embryo was 

 then enlarged on millimeter paper, to the scale of the drawings of 

 sections, and the aortic arches were plotted with the aid of pro- 

 portional dividers. These reconstructions represent projections of 

 the lumina of the blood-vessels, and in all cases were corrected, for 

 shrinkage of the embryo in paraffin, so that the measurements on 

 any particular section, compared accurately with those on the re- 

 constructions. 



A. The aortic arches in rabbit embryos. 



In the discussion of each embryo the discription is followed by 

 a brief summary and comment. 



Embryo of the ninth day (first half). The age of this 

 embryo is uncertain since no record was preserved, but it was 

 estimated to be in the first half of the ninth day. In surface view, 

 Fig. 1, Plate 22, the embryo shows the outline of the mandibular 

 pouch. The specimen as a whole, was in imperfect condition, but 

 the blood-vessels could de satisfactorily reconstructed to the region 

 of the heart. A single pair of aortic arches is present, lying in the 



