ON THE ORIGIN OP VERTEBRATES PROM ARACHNIDS. 3G9 



derm accelerated, we might then have a continuous sheet of 

 mesoderm and endoderm, apparently a forward continuation of 

 the anterior wall of the terminal pore or telopore (Fig. 16, 

 D and f). Suppose the hsemal edges of the mesodermic bands 



""-"^^^^^^ 



Fig. 16. — Diagrams to illustrate the formation of germ layers. — A. Cross 

 section of the posterior end of telopore of an insect just before it closes. 

 B. Section a little in front of the last, showing the cords of mesoderm 

 and endoderm. C. One still farther forward. D. Section through an 

 embryo in which it is supposed mesoderm and endoderm form a continuous 

 layer. E. Section of an embryo in which it is supposed the lateral 

 endodermic bands have grown completely round the yolk before the 

 mesoderm became segmented and separated from the endoderm. F, G, H, 

 Three longitudinal sections, showing successive stages in the formation 

 of a telopore by the invagination of teloblasts. F. Insect. G. Crus- 

 tacean, Astacus. iy. Amphioxus. 



unite before the mesoderm is separated from the endoderm, 

 we should have a condition like that in Amphioxus, where 

 the endoderm and mesoderm form a sac opening outward 

 posteriorly by the telopore (Fig. 16, e and h). All the 

 so-called gastrulas and blastopores of Arthropods aud Verte- 

 brates can be explained on the same principle. Accepting 



