590 DEVELOPMENT OF CIRCULATING APPARATUS. 



arterial trunk that springs from the ventricle, divides into a < 

 set of arches on each side (fig. 326), which closely resemble 

 the branchial arches of Fishes and Tadpoles. Although 

 no gills are present, yet there is a series of clefts on 

 each side of the neck, passing through to the pharynx, 

 which are analogous to the branchial apertures of the 

 cartilaginous Fishes ( 317). After a time, however, the 

 auricle and ventricle of the heart are each divided by a 

 vertical partition, so that four cavities are formed, out of 

 the two which previously existed ; and at the same period, 

 the arrangement of the vessels undergoes a change, by the 

 division of some trunks, and the obliteration of others, so 

 that they gradually assume the distribution which is charac- 

 teristic of warm-blooded animals ( 281). But even up to 

 the time of the birth of the Mammalia, there is a communica- 

 tion between the two sides of the heart, and between the 

 pulmonary and systemic vessels, which is closely analogous to 

 that which permanently exists in the Crocodile ( 283). 



763. Again, the space within the head of the embryo, into 

 which the vertebral canal widens-out ( 757), is occupied in 

 the first instance by a succession of vesicles or bags, arranged 

 in a linear series (fig. 323, d, e, /) ; each of which is the rudi- 

 ment of one of those principal ganglionic masses, that col- 

 lectively make-up the brain of the Fish ( 453), in which 

 they present a very similar aspect (fig. 192). As in many 

 Fishes, too, the Cerebrum is inferior in size to the Optic 

 ganglia, and only comes to surpass and finally (as it were) to 

 overpower them ( 455, 456) in the later periods of embry- 

 onic development. 



764. The true representation of these and similar facts is 

 not, as was maintained when they were first brought into view, 

 that the several organs of the higher animals go through a 

 series of forms which remain permanent in the lower; but 

 that the development of all animals formed upon the same 

 general plan commences in the same manner, their special 

 differences manifesting themselves as development proceeds. 

 Thus, as we have seen, the foundation of the Vertebral column 

 is laid in all Vertebrata in precisely the same method ( 757) ; 

 in some of the lowest Fishes, the evolution of this structure 

 is checked at so early a period that it never advances beyond 

 the embryonic type ( 53) ; but the fully-formed spine has a 



