392 C. 0. WHITMAN. 



two halves of the ring coalesce in the same manner as in 

 Clepsine. 



Let us now consider the two theories of embryo-formation in 

 a more general way. The theory of concrescence undertakes 

 to give a rational explanation of a very large body of pheno- 

 mena; while that of differentiation cannot even make the 

 slightest pretension to anything of the kind. What is differ- 

 entiation except an undefined mode of growth and develop- 

 ment ? It is not a mechanical explanation, but simply a vague 

 name for unknown processes. This theory, even when de- 

 fended by the sagacity of Balfour, has never offered a sugges- 

 tion by way of explaining the germinal ring, and the uniform 

 relations which this ring sustains to the embryo. It cannot 

 tell us why all the so-called " phenomena of concrescence^^ are 

 situated behind the embryo, instead of before it; nor can it 

 give any explanation of the general nature of these phenomena, 

 except when it goes a begging by calling in the aid of the con- 

 crescence theory. On the other hand, the theory of concre- 

 scence makes known a law of formation which may, with 

 reason, be said to hold good throughout the vertebrate group, 

 and their nearest invertebrate allies. So far as these allies are 

 concerned, the law is an established fact, about which there is 

 not, and cannot be, any controversy ; and among the lower 

 vertebrates the appearances are unquestionably in its favour. 

 Assuming that the immediate ancestor of the Vertebrata was a 

 segmented worm, it is evident that our theory of the formation 

 of the embryo should include both groups of animals. The 

 fundamental agreement, which we should naturally expect to 

 find, appears first of all in the formation of a germinal 

 ring, composed of two symmetrical halves, which 

 coalesce along the median neural line from before 

 backward, thus producing a bilateral embryo. So 

 far there is essential agreement in the two great classes of 

 segmented worms, the Cha)topods and Leeches. 



I tliiiik no exception to tliis statement can be justly taken on the ground 

 that the germ-bands in some cases contain neuroblastic as well as mcsoblastic 

 elements. J3ut as Balfour has laid considerable stress on this very point, and 



