398 Harriet Lehmakn, 



dorsal aorta, and a secondary subclavian of later formation coming from 

 the ventral portion of the third arch. The latter becomes the definitive 

 subclavian. In mammals the arteries to the fore-limbs first arise from the 

 dorsal aorta, but owing to a later symmetrical splitting in this vessel (like 

 Anura), the subclavians eventually spring from the lengthened portions of 

 the two aortic roots. 



F. T. Lewis presented a preliminary paper before the "Association 

 of American Anatomists", December, 1903, on "The Intraerabryonic Blood- 

 vessels of Rabbits from eight and one-half to thirteen Days", in the 

 published abstract of which appears the statement that "The irregular 

 small arteries around the fourth entodermal pouch do not, as Zimmermann 

 believed, form a distinct aortic arch." Lewis has also published a re- 

 construction of the aortic arches in a pig of 12 mm.^) 



A survey of the literature shows, that while a considerable number 

 of contributions have been made toward a knowledge of various stages in 

 the history of the aortic arches in vertebrates, comparatively few of these 

 represent observations upon a close series of embryos. This is especially 

 true as regards birds and mammals. Rathke, His and Tandler have 

 covered in a critical way, somewhat extended periods in the history of 

 the aortic arches in certain mammals, but further observations based on 

 connected series of embryos, are still to be desired for this class of verte- 

 brates. 



Descriptiye part. 



The following- observations were undertaken at the sug-gestion 

 of Professor William A. Locy, and were carried on during- 1902 — 1904, 

 in the Zoological Laboratory of Northwestern University. I wish 

 to acknowledge my indebtedness to Professor Loot for his able 

 direction in the course of the work, and for his painstaking- criticism 

 of the manuscript. The work was completed after the writer was 

 appointed to the Alice Freeman Palmer Fellowship of Wellesley 

 College. 



Material, etc. Both pig and rabbit embryos were used. The 

 youngest pig embryos obtainable from the packing houses were of 

 nearly twenty days development, and the series used begins, there- 

 fore, at that age. The collection of rabbit embryos embracing earlier 

 stages, was obtained through the courtesy of Dr. C. S. Minot. This 

 series begins with an early stage of the ninth day, and extends 

 through twelve and one-half days. The two series supplement one 

 another and overlap: the rabbits exhibit especially well stages in 



1) Minot, A laboratory text-book of embryology, 1903, 



