136 TIIK EMBRYOLOGICAL CRITERION 



In his view of the metamorphosis of the aortic arches in 

 the chick the first two pairs disappeared completely, the third 

 pair gave rise to the arteries of the head and the fore-limbs, 

 the right side of the fourth arch became the aorta, the left half 

 of the fourth and the right half of the fifth arch became the 

 pulmonary arteries, while the left half of the fifth arch dis- 

 appeared. This schema, which for the last three arches was the 

 same as Huschke's, von Baer upheld for the chick even in the 

 second volume of his Entwickelungsgeschichte (p. 1 16) ; he recti- 

 fied it, however, for mammals in the same volume (p. 212), 

 deriving both pulmonary arteries from the fifth arch, and the 

 aorta from the fourth left. He fully recognised the great 

 analogy of the embryonic arrangement of gill-arches and 

 gill-arteries in Tetrapoda with their arrangement in fish 



(i-, PP- 53, 73)- 



Huschke, in a paper of I832, 1 chiefly devoted to the 



development of the eye, figured and described the developing 

 upper and lower jaws, and maintained against von Baer that 

 the first slit turns into the auditory meatus and the Eus- 

 tachian tube. 



These were the first papers of the embryological period. 

 Before going on to discuss the principles which guided 

 embryological research during the next ten or twenty years 

 it is convenient to note what were the main lines of work 

 characterising the period. 



The typical figure of the period is Rathkc, who produced 

 a great deal of first-class embryological work. He was, even 

 more than von Baer, a comparative embryologist, and there 

 were few groups of animals that he did not study. His first 

 large publication, the ]>citriigc :jnr Gi-sc/iic/ite dcr Thicncclt 

 (i.-iv., Halle, 1820-27), contained much anatomical work in 

 addition to the purely embryological ; he commenced here 

 his series of papers on the development of the genital and 

 urinary organs, continued in the Abhandlungen zur lUldinigs- 

 nnd Entwickelungs-Geschichte dcs Mcnschcn mid dcr TJiicrc 

 (i., ii., Leipzig, 1832-3). A fellow-worker in this line was 

 Johannes Miiller, whose Bildungsgeschichte dcr (icn it alien 

 (Diisseldorf) appeared in iS3o. 



In a memoir on the development of the crayfish which 

 1 Meckel's Arc/u'v, vi., pp. 1-47, 1832. 



