122 BIOLOGICAL LECTURES. 



lematical, and the evidence drawn from it as therefore unsatis- 

 factory. It would be useless in such a case to urge the point, 

 and also wholly needless, as examples abound that are not open 

 to such objections. 



The form-changes by which the fish blastodisc passes into 

 the germ-ring stage are examples of this kind. It is well 

 known that the transformation of the blastodisc just before 

 the appearance of the germ-ring is quite rapid, at least in the 

 pelagic fish-egg, and also quite independent of cell-formation. 

 The discoidal germ-ilfass suddenly thins out, but not uniformly 

 in all parts. The half of the disc in which the embryo is to 

 be formed remains thick, anticipating as it were the axial con- 

 centration which is to follow, while the half lying in front of 

 this is rapidly reduced to a thin epithelial membrane. This 

 regional differentiation of the outer layer and the concomitant 

 formation of the germ-ring, including the forward movement 

 of the embryonic plate ("head process"), which advances in 

 an axial direction to the very centre of the disc, are indubi- 

 tably accomplished, not by the aid of cell-formation, but by 

 formative processes of an unknown nature, but nevertheless 

 real and all-controlling. Cell-formation, to be sure, goes on, 

 but it seems to me certain that it has no directive influence 

 on the formative processes. The cleavage runs on from begin- 

 ning to end, regularly or irregularly, without modifying in 

 any essential way the form of the blastodisc. All at once, 

 when this segmentation has been carried to a certain point, 

 the transformation sets in and goes rapidly on, without inter- 

 rupting cell-formation, but to all appearance quite indepen- 

 dently of it. 



In the axial concentration of the very broad embryonic plate 

 we see a formative process that can have nothing whatever to 

 do with cell-division. Again, in the establishment of the caudal 

 end of the embryo, long before that part of the germ-ring which 

 represents, historically at least, this end can be brought into 

 place, we have another decisive test of formative power assert- 

 ing itself, not only independently of cell-division, but also 

 against all the obstructions interposed by the yolk. This pre- 

 potency of the "plastic power" (Schwann) is seen to great 



