5 i 8 
Phenomena of Ontogenesis 
[October, 
formation of new cavities in the heart, and by the production 
of new vessels. The clefts between the branchial arches 
close up and disappear. In the amphibians the branchial 
arches really bear gills during the larval state. A temporary 
respiratory apparatus, the allantois, bearing a strong resem- 
blance in its own character, and especially in its vascular 
connections, to the gills of the Mollusca, is developed at an 
early stage : it seldom attains any size in Mammalia, being 
early superseded by another provision for the aeration of the 
blood. Many of the malformations occasionally observed 
in man are for the most part due to arrest of development ; 
the circulating apparatus is sometimes permanently fixed in 
conditions that are properly characteristic of cold-blooded 
animals : and it is interesting to remark, too, that the va- 
rieties which not unfrequently present themselves, in the 
arrangements of the principal trunks given off from the 
aorta, find their analogues in the arrangements that are 
normally characteristic of some one or other of the Mam- 
malia. The venous system of the human body undergoes 
changes which are even more remarkable than those of the 
arterial trunks. In its earliest condition it has been ascer- 
tained by Rathke to present essentially the same type in the 
embryos of all vertebrated animals, the peculiarities of each 
group being acquired by a process of subsequent transform- 
ation. There is at first a pair of anterior venous trunks, 
receiving the blood from the head, and a pair of posterior 
trunks, formed by the confluence of the veins of the trunks, 
of the Wolffian bodies, &c. : the former are persistent as 
the jugular veins; the latter remain separate in most fishes, 
where they are designated the Cardinal Veins ; but in man 
(as in warm-blooded Vertebrata generally) they are only re- 
presented by the venae azygos, major and minor, which 
coalesce into a common trunk for a considerable part of 
their length. One of the anterior trunks and one of the 
posterior unite on either side to form a canal, which is known 
as the DuCtus Cuvieri ; and the duCts of the two sides 
coalesce to form a shorter main canal, which enters the 
auricle, at that time an undivided cavity. This common 
canal is absorbed into the auricle at an early period in all 
Vertebrata above fishes ; and after the septum auriculorum 
is formed, the two Cuvierian dudds separately enter the right 
auricle. This arrangement is persistent in birds and the 
inferior mammals, in which we find two vense lavse supe- 
riors entering the right auricle separately ; but in the 
higher Mammalia and in man the left dudd is obliterated, 
and the right alone remains as the single vena lava superior, 
