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, aud 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 pala30stracan, are all shown with vivid clearness 

 by Kupffer's investigations of the early stage of Ammocoetes, 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 branchiae arise as 



