458 THE ORIGIN OF VERTEBRATES 
diverticula, it points not only to an arthropod ancestry, but to a 
descent from a particular group of primitive arthropods. Then 
comes the formation of the cerebral vesicles, and the formation of 
the optic cup, telling us as plainly as can be how the invasion of 
nervous material over this simple cephalic stomach and its diverticula 
has altered the shape of the original tube, and more and more 
enclosed it with nervous elements. 
So, too, in the spinal cord region. When the tube is first formed, 
it is a large tube, the latero-ventral part of which presents two 
marked bulgings; connecting these two bulgings is the anterior 
commissure. These two lateral bulgings, with their transverse 
commissure, represent, with marked fidelity, the ventral ganglion- 
masses of the arthropod with their transverse commissure, and occupy 
the same position with respect to the spinal tube, as the ganglion- 
masses do with respect to the intestine in the arthropod. Then the 
further development shows how, by the subsequent growth of the 
nervous material, the calibre of the tube is diminished in size, and 
the spinal cord is formed. 
Again, I say, is it possible to conceive that embryology should 
indicate the nature of the origin of the vertebrate nervous system 
more clearly than it does ? 
It is the same with all the other organs. Take, for instance, the 
skeletal tissues. The study of the vertebrate embryo asserts that the 
cartilaginous skeleton arose as simple branchial bars and a simple 
cranio-facial skeleton, and also that the parenchymatous variety of 
cartilage represents the embryonic form. Word for word, the early 
embryonic stage of the vertebrate skeleton closely resembles the 
stage reached in the arthropod, as shown by Limulus, and again 
records, unmistakably, the past history of the vertebrate. 
So, too, with the whole of the prosomatic region ; the situation 
of the old mouth, the manner in which the nose of the cephalaspidian 
fishes arose from the paleostracan, are all shown with vivid clearness 
by Kupffer’s investigations of the early stage of Ammoccetes, while 
at the same time the closure of the oral cavity by the septum shows 
how the oral chamber was originally bounded by the operculum. 
. Nay, further, the very formation of this chamber embryologically was 
brought about by the forward growth of the lower lip, just as it must 
have been if the chilaria grew forward to form the metastoma. 
So, too, the study of the embryo teaches that the branchie arise as 
