66 Evolution and Adaptation 
have added to the cartilaginous skull certain plates in the 
dermal layer of the skin. In the higher forms we find the 
skull composed of two sets of bones, one set developing from 
the cartilage of the first-formed cranium, and the other having 
a more superficial origin; the latter are called the membrane 
bones, and are supposed to correspond to the dermal plates 
of the ganoids. 
In the development of the kidneys, or nephridia, we find, 
,perhaps, another parallel, although, owing to recent dis- 
coveries, we must be very cautious in our interpretation. As 
yet, nothing corresponding to the nephridia of amphioxus has 
been discovered in the other vertebrates. Our comparison 
must begin, therefore, higher up in the series. In the sharks 
and bony fishes the nephridia lie at the anterior end of the 
body-cavity. In the amphibia there is present in the 
young tadpole a pair of nephridial organs, the head-kidneys, 
also in the anterior end of the body-cavity. Later these are 
replaced by another organ, the permanent mid-kidney, that 
develops behind the head-kidney. In reptiles, birds, and 
mammals a third nephridial organ, the hind-kidney, develops 
later than and posterior to the mid-kidney, and becomes the 
permanent organ of excretion. Thus in the development of 
the nephridial system in the higher forms we find the same 
sequence, more or less, that is found in the series of adult 
forms mentioned above. The anterior end of the kidney 
develops first, then the middle part, and then the most poste- 
rior. The anterior part disappears in the amphibians, the 
anterior and the middle parts in the birds and mammals, so 
that in the latter groups the permanent kidney is the hind- 
kidney alone. 
The formation of the heart is supposed to offer certain 
parallels. Amphioxus is without a definite heart, but there is 
a ventral blood vessel beneath the pharynx, which sends blood 
to the gill-system. This blood vessel corresponds in position 
to the heart of other vertebrates. In sharks we find a thick- 
