6 ON VERTEBRATE ANIMALS 



afterwards unite above and enclose the spinal marrow. Beneath, 

 this layer bends itself round in order to form the abdominal cavitj. 

 The mucous layer gives origin to the intestinal canal. 



It is worthy of remark that the position of the embryo with 

 respect to the yolk differs from that in the articulate and other 

 invertebrate animals. In these the yolk lies on the dorsal surface, 

 in the vertebrate animals always on the abdominal surface of the 

 embryo. The central portion of the germ, from which the develop- 

 ment begins, is, in vertebrate animals, the dorsal part. Hence it 

 arises also, that if we suppose the intestine to be the axis by which 

 the body is divided longitudinally, the principal mass of the nervous 

 system, the stem — the brain and spinal marrow — lies above this 

 axis, whilst the gangliated cord of articulate animals which corre- 

 sponds to the spinal marrow lies under its axis, on the abdominal 

 surface; the spinal marrow is here replaced by an abdominal cord. 

 Inversely the heart in vertebrate animals lies below, in invertebrate 

 animals above the same axis\ We have seen above that the 

 dorsal vessel of insects takes in them the place of a heart (compare 

 Vol. I. p. 258). 



The position of the central part of the nervous system (the 

 spinal cord and brain) on the dorsal surface, and its enclosure in a 

 special cavity distinct from that of the viscera, form two principal 

 characters of vertebrate animals. If we follow the branches from 

 which the nerves of the skin or those of the muscles arise to their 

 origin in larger and still larger trunks, then we find that they all run 

 towards the brain or the spinal marrow, and lose themselves in the 

 substance of these parts. Brain and spinal marrow are consequently 

 the central parts of the nervous system of animal life. They are, 

 however, only two divisions of one and the same whole, of which 

 the development and magnitude are usually inversely proportional 

 to each other. In the most perfect vertebrate animals, namely the 

 mammals and especially in man, the mass of the brain far surpasses 

 that of the spinal cord. 



In the brain and in the spinal cord the nerve-substance 

 appears under two modifications. Chemical investigation has 



^ Compare H. Rathke Untersuchungen iiher die Bildung und Entwiclcelung des 

 Flusskrcbses. Leipzig, 1829, s. 77—90; see also Baer Be Ovi Mammal ct Ilominis 

 genesi, p. 24. 



