407 



«nee in reptiles and the chick of a rudimentary arch between the 

 fourth and supposed fifth. Subsequent observations, notably those of 

 ZiMMEKMANN (1889) and Tandler (1902), have led to the recognition 

 of six aortic arches in mammals. The discovery of a rudimentary 

 fifth arch in mammals makes the number of aortic arches for Amniota 

 the same as in Dipnoi and Amphibia, and establishes an identity, as 

 regards the place of origin of the pulmonary artery, in all lung- 

 breathing vertebrates. The diagrams of Boas have accordingly re- 

 placed those of Rathke. 



The following descriptions of reconstructions of the aortic arches 

 in rabbit and pig embryos are taken from a larger paper dealing with 

 the subject, soon to be published in another place. In that paper, 

 the literature is reviewed and the descriptions given in greater detail. 

 The observations were carried on during 1902 — 1904, in the Zoological 

 Laboratory of Northwestern University. I wish to acknowledge my great 

 indebtedness to the Director, Professor Loct, for selecting the subject 

 and for his supervision of the research. 



The researches of the following named observers all bear upon 

 the subject, and receive notice in my larger paper: v. Baer (1827, 

 '28, '37); Huschke (1828); Thompson (1830); Rathke (1843, '57); 

 Sabatier (1874); Boas (1881, '82, '86, '87); His (1880—85; van 

 Bemmelen (1886); Mackay (1888); Mall (1888, '91); Zimmermann 

 (1889, '90); Marshall and Bles (1890); Hochstetter (1890, '91, 

 1903); Broom (1898); Tandler (1902); Bremer (1902) and F. T. 

 Lewis (1903). 



Material. 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, therefore, 

 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 the form- 

 ation 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. 



A. The Aortic Arches in Rabbit Embryos. 



The reconstructions illustrated in this paper represent carefully 

 measured projections of the lumina of the blood-vessels, and were 



