654 STRUCTURE AND GROWTH OF DOMESTICATED ANIMALS. 
widens sidewise on its anterior part, and rises a little from the 
rest. In one word, this cavity forms the channel for the spinal 
marrow, and its front part the cavity for the brain, and the walls 
grow to be flesh and bone to form the dorsal spine. The upper 
part represents the axis of the skeleton, with the surrounding soft 
parts: the lateral parts form the ribs with their fleshy covering, 
and, the animal thus closing over the yolk, we have the abdomi- 
_ nal cavity. Now, it requires a little more enlargement, a little 
more change into different substances, to complete the formation 
of the new being. The gelatinous substance outside the main axis 
is changed into a fibrous structure, which is muscle. The little 
opaque bodies in the axis and upon its sides absorb some earthy 
material contained in the primitive substance from which they have 
arisen, and thus bone isformed. The fluid in the upper cavity be- 
comes a little more granular and more solid, and it is the brain 
and spinal marrow. The yolk is absorbed during the process of 
growth, but the wall within which it is contained is elongated and 
enlarged, and in consequence of farther changes in the substance 
of that part of the yolk which is in immediate contact with the 
body-walls, the alimentary cavity is formed. You have, in fact, all 
the organs of the animal growing in the same way, by successive 
transformations of the homogeneous mass into all the various tis- 
sues and organs which build up the animal in its perfect condition. 
From the time the chick has reached the condition in which al 
its organs are fairly sketched, it simply grows larger and larger, 
and finally breaks through the shell. The skin has already become 
distinct from the muscles; the feathers begin to be formed, and all 
those parts with which you are familiar may readily be distin- 
guished. You see now by what complicated process (the details 
of which I have considerably abridged) this is brought about. 
I have given you but a meagre outline of the changes which take 
place in the formation of quadrupeds, birds, reptiles, and fishes, 
though this may be sufficient to show that these processes must be 
studied in every animal independently. 
The figures on the following page, representing a fish in the 
egg, show at once how different the growth of these animals 1s 
from that of the mammalia and birds. Here we have no amnios ; 
the young fish remains free upon the surface of the yolk. The 
structure of the body, however, and the circulation of the blood 
upon the yolk, are strikingly similar to those of the dog, the 
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