iSyy . ] and the Evolution Hypothesis . 519 
a transverse communicating branch being formed, to bring 
to it the blood of the left side. The double vena lava some- 
times presents itself as a monstrosity in the human subject; 
some vestiges of the original arrangement may be traced 
even in the normal condition of the venous system in the 
adult. The embryological development of the urinary 
organs in vertebrated animals is a subject of peculiar inte- 
rest, owing to the correspondence which may be traced 
between the transitory forms they present in the higher 
classes and their permanent conditions in the lower. 
Vessels exactly analogous to the corpora malpighiana of 
the true kidney remain as the permanent urinary organs 
of fishes, but in the higher Vertebrata they give place 
to the true kidneys. At the end of the third month 
(in the human subject) the kidneys consist of seven or eight 
lobes ; their excretory duCts still terminate in the canal 
which receives those of the Wolffian bodies and of the 
Fallopian tubes ; and this opens, with the reCtum, into a 
sort of cloaca, analogous to that which is permanent in the 
oviparous Vertebrata. In the embryo the distinction of 
sexes is altogether wanting at first, and the conformation of 
the external parts of the apparatus is originally the same 
in man and the higher Mammalia as it permanently is in 
the oviparous Vertebrata. A partial representation of the 
phase of development at the tenth or eleventh week in the 
human subject is found in the permanent condition of the 
Struthious birds and of the implacental Mammalia. In the 
small proportion which the cerebral hemispheres bear to the 
other parts, in the absence of convolutions, in the deficiency 
of commissures, and in the general simplicity of structure 
of the whole, there is a certain correspondence between the 
brain of the human embryo at about the sixth week and 
that of a fish. About the twelfth week we find the cerebral 
hemispheres much increased in size, and arching back over 
the thalami and corpora quadrigemina ; still, however, they 
are destitute of convolutions, and are imperfectly connected 
by commissures, and there is a large cavity yet existing in 
the corpora quadrigemina which freely communicates with 
the third ventricle. In all these particulars there is a strong 
analogy between the conditions of the brain of the human 
embryo at this period and that of the bird. Up to the end 
of the third month the cerebral hemispheres present only 
rudiments of anterior lobes, and they do not pass beyond 
that grade of development which is permanently character- 
istic of the marsupial Mammalia, the thalami being still 
incompletely covered in by them. During the fourth and 
