THE PRINCIPLES OF EMBRYOLOGY 483 



therefore be called hypoblast, spreads continuously over the yolk, as 

 also does the surface epithelial layer, or epiblast. Such spreading 

 is always a continuous one for both surfaces, so that the yolk is 

 gradually enclosed by a continuous orderly growth from the germ- 

 band, and not by the settling down of free cells in the yolk here 

 and there to form the gut-lining. This steady orderly development 

 proceeds owing to the nourishment afforded by the activity of the 

 free cells or vitellophags and the absorbing power of the hypoblast, 

 a steady growth round the yolk which results in the formation of the 

 gut-tube, the outer covering and all the muscular and excretory 

 organs. Where, then, is this starting-point, this germ-band from 

 which the whole embryo grows ? It forms the mid ventral area of 

 the adult animal, it corresponds exactly to the position of the 

 central nervous system. The whole phenomenon of embryonic 

 growth in the scorpion is exactly what must take place on the 

 argument deduced from the study of the adult that the animal 

 arises as a neuro-epithelial syncytium, and we see that that layer of 

 cells which is situated next to the food-material forms the alimen- 

 tary tube. It is not a question whether such layer is ventral or 

 dorsal to the neural cells, but whether it is contiguous to or removed 

 from the food-material. 



Take, again, a meroblastic vertebrate egg as of the bird. Again we 

 find free cells passing into the yolk to act as vitellophags, the so-called 

 periblast cells ; again we see that the embryo starts from a germ- 

 band or embryonic shield, and spreads from there continuously and 

 steadily ; again we see that the layer of cells which lies against the 

 yolk absorbs the fluid pabulum for the growing cells ; again we see 

 that the area from which the whole process of growth starts is that 

 of the central nervous system, and again we see that those cells 

 which are contiguous to the food form the commencing gut, and are 

 therefore called hypoblast, though in this case they are ventral not 

 dorsal to the neural layer. 



The comparison of these two processes shows that there is one 

 common factor, one thing comparable in the two, one thing that is 

 homologous and is the essential in the formation of that part of the 

 animal which I have called the host, and that is the central nervous 

 system. Whether the epithelial layer which lies ventrally to it or the 

 one that is dorsal forms the gut depends upon the position of the 

 food-mass. Where the food is, there will be the absorbing layer. 



