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,f); 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 
