HOMOLOGIES OF A LOBSTER. 53 



that the primitive trace of the embryo and the central 

 organ of the nervous system on the one hand, and 

 the blastodermic vesicle on the other, do not differ 

 in their topographical relationship to each other from 

 the corresponding parts in the ovum of the vertebrate 

 animal. Putting out of view now the vertebral 

 column and more complete development of the central 

 organ of the nervous system of the vertebrate animal, 

 it may be said that the subsequent difference between 

 the vertebrate and arthropod is that the aspect of 

 the body where the primitive trace and the central 

 organ of the nervous system appeared, becomes in 

 the vertebrate animal what is known as the dorsal 

 aspect or back of its body, and in the arthropod what 

 is commonly called the ventral aspect of its body or 

 belly. Fundamentally, however, the dorsal aspect 

 or back of a vertebrate animal, is homologous with 

 the so-called ventral aspect or belly of a lobster or 

 insect ; and we shall be correct in saying that the 

 real difference between vertebrate and arthropodous 

 animals is — not that the central organ of the nervous 

 system or spinal marrow is situated along the back 

 in the vertebrate, and along the belly in the arthro- 

 podous animal ; but that in their natural prone 

 position, vertebrate animals have their vertebral 

 aspect or back directed upwards, whereas the natural 

 position of arthropodous animals is the reverse of 

 this, being that in which the vertebral aspect of their 

 body is downwards — the aspect in them commonly 

 called the back being in reality, as I have before 



