THE BLOOD-COEPTTSCLES. 99 



vessels of the liigliest Vertehrata undergo during embryonic 

 life is exceedingly remarkable, and is repeated as we ascend 

 in the series of adult vertebrates. 



At first, the heart of a mammal lies under the middle of the 

 head, immediately behind the first visceral arches, in which 

 the first pair of aortic arches ascend. As the other pairs of 

 aortic arches are developed the heart moves backward ; but 

 the fourth pair of aortic arches, by the modification of one 

 of which the persistent aorta is formed, lies, at first, no 

 farther back than the occipital region of the skull, to which, 

 as we have seen above, the fourth pair of visceral arches 

 belongs. As the two pairs of cornua of the hyoid belong 

 to the second and the third visceral arches, the larynx is 

 probably developed within the region of the fourth and 

 fifth visceral arches ; hence, the branches of the pneumo- 

 gastric, with which it is supplied, must, oi-iginally, pass 

 directly to their destination. But, as development pro- 

 ceeds, the aortic arches and the heart become altogether 

 detached from the visceral arches and move back, until, at 

 length, they are lodged deep in the thorax. Hence the 

 elongation of the carotid arteries ; hence also, as the larynx 

 remains relatively stationary, the singular course, in the 

 adult, of that branch of the pneumogastric, the recurrent 

 laryngeal, which primitively passed to the laryngeal region 

 behind the fourth aortic arch, and consequently becomes 

 drawn oiit into a long loop — the middle of it being, as it 

 wei'e, pulled back, by the retrogression of the aortic arch 

 into the thorax. 



Tlie Blood Corpuscles. — Corpuscles are contained in the 

 blood of all Vertehrata. In Amphioxus they are all of one 

 kind, colourless and nucleated. The genus Leptocejplialus, 

 among the Teleostei, is said to possess the same pecu- 

 liarity ; but in all other known Vertehrata, the blood con- 

 tains corpuscles of two kinds. 



In IchtJiyopsida and Sauropsida, both kinds are nu- 

 cleated; but one set are colourless, and exhibit amseboid 

 movements, while the others are red, and do not display 



