482 THE ORIGIN OF VERTEBRATES 



at the problem. First, the evolution of the animal kingdom means 

 essentially the evolution of the host, for that is what forms the 

 individual ; secondly, as the host is composed of a syncytium, the 

 common factor of whose elements is the neural moiety, it follows 

 that the tissue of central importance for the evolution of the host 

 must be, as indeed it is, the nervous system. Further, seeing that 

 the growth of the individual means the orderly spreading out of the 

 epithelial moiety away from the neural moiety, it follows that the 

 germ-band or germ-area from which growth starts must be in 

 the position of the nervous system. If then, the nervous system in 

 the animal is a concentrated one, then the growth will emanate 

 from the position of such nervous system. If, on the other hand, 

 the nervous system is diffused, then the growth will also be diffused. 



In this book I have throughout argued that the ancestors of verte- 

 brates belonged to a great group of animals which gave origin also to 

 Limulus and scorpion-like animals ; it is therefore instructive to see 

 what is the nature of the development of such animals. For this 

 purpose I will take the development of the scorpion, as given by 

 Brauer, for he has worked out its development with great thorough- 

 ness and care. His papers show that the segmentation is discoidal, 

 and results in an oval blastodermic area lying on a large mass of yolk. 

 Very early there separates out in this area genital cells and yolk- 

 cells, which latter move freely into the yolk and prepare it into a 

 fluid pabulum for the nutrition of the cells of the embryonic shield 

 or germ-band. These free yolk-cells do not take part in the formation 

 of the germinal layers, nor does the endoderm when formed give 

 origin to free yolk-cells. 



The cells of the germ-band form a small compact area, in which 

 by continual mitosis the cells become more than one-layered, and soon 

 it is found that those cells which lie close against the fluid pabulum 

 form a continuous layer and absorb the nutritious material for them- 

 selves and the rest of the embryo. While this area is thus increasing 

 in thickness by continuous development, the group of genital cells 

 remains always apart, increasing in number, but being always in a 

 state of isolation from the cells of the rest of the growing area. Thus 

 from the very first Brauer's observations on the development of 

 the scorpion point to the formation of a syncytial host containing 

 separate genital cells. The continuous layer of cells against the 

 fluid pabulum, which is already functioning as a gut, and may 



