LIKXr.AX SOCTETT OF LONUOX. 1 3 



<he law which lays down that the past phylogeuetic stages which 

 have led to the evolution of any individual are indicated to some 

 extent in the ontogeny of that individual. 



This law is contirmed and indicated in a most amazing way by 

 my theory. The theory asserts that the clue to the origin of 

 Vertebrates is to be found in the tubular nature of the central 

 nervous system of the Vertebrate in that the central nervous 

 system is in reality formed of two things : (1 ) a central nervous 

 system of the Arthropod type, and (2) an epithelial tube in the 

 position of the alimentary canal of the Arthropod. 



Is it possible for embryology to recapitulate such a phylogenetic 

 history more clearly than is here the case? In order to avoid all 

 possibility of our mistaking the clues, the nerve-tube in the embryo 

 always opens into the anus at its posterior end, while in the larval 

 Amphioxus it is actually still open to the exterior at its anterior 

 end. Consider the shape of the nerve-tube when first formed in 

 the A^ertebrate. At the cephalic end a simple bulged-out tube with 

 two simple anterior diverticula, which passes into a narrow straight 

 spinal tube; from this large cephalic bulging a narrow diverticulum, 

 the infundibulum, passes to the ventral surface of the forming 

 brain. This tube is the embryological expression of the simple 

 dilated cephalic stomach, with its ventral oesophagus and two 

 anterior diverticula, which opens into the straight iutestine of the 

 arthropod. Nay more, by its very shape and the invariable 

 presence of two anterior 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 trans- 

 verse commissure, represent with marked fidelity the ventral 

 ganglion masses of the Arthi-opod with their transverse commissure, 

 and occupy the same position with respect to the spinal tube, as 

 the gangliou-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 example 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 



