408 



plotted as follows: A camera sketch was made of each embryo to be 

 sectioned, the plane of the sections 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 the sections, and 

 the aortic arches were plotted with the aid of proportional dividers. 

 Embryo of the ninth day (first half). Fig. 1, represents 

 a reconstruction of the aortic arches of the left side of a rabbit 

 embryo, estimated to be in the first half of the ninth day of devel- 



a d. 



Fig. 1. Reconstructiou of the Aortic arches in Rabbit embryo of 9th day (first 

 half). / first arch, //, //' buds of second arch, a.d. dorsal aorta, aud. auditory vesicle. 

 X about 95. 



opment. The first arch running in the mandibular pouch is present. 

 Its walls are reticulated and the interior of the vessel, in its upper 

 part, is crossed by trabeculse. Two small spurs, one on the base of 

 the aortic arch (JZ), and one on the dorsal aorta {H')^ are supposed 

 to represent the beginnings of the second aortic arch. The aortae in 

 this specimen are separate throughout their course. 



Embryo of the ninth day (second half). A later stage — 

 estimated to be in the second half of the ninth day — is represented in 

 Fig. 2. The arches present are, a first pair, an incomplete second, 

 and a third pair, complete upon the left side, but incomplete upon 



