58 THE PHYSIOLOGY OF TWINNING 



e) As soon as this Knopf region is organized the axis 

 of the embryo is complete and all increase in the length 

 of the axis occurs in a growing zone between the head and 

 the tail-bud. 



That this is not merely one man's interpretation of 

 these conditions is seen in the fact that essentially the 



Figs. 32, 2;^. — Diagrams showing Kopsch's interpretation of the 

 mode of embryo formation in the teleost fishes. K, the Kopf; R, the 

 Knopf. For explanation see text. (From Kopsch.) 



same position is taken by Morgan, Virchow, Sumner, 

 Jablonowski, and other workers on teleost embryology. 

 Professor F. R. Lillie, who has given much attention to 

 this matter, considers the position of Kopsch entirely 

 sound. I propose then to accept this consensus of 

 judgment about the nature of concrescence and the 

 formation of the embryo in teleosts. Kopsch's conclu- 

 sions on these problems are as follows. 



When the embryonic primordium is still merely a 

 somewhat thicker sector than the germ ring (Fig. 32), 

 there is a middle region (K) the Kopf (or head) and two 

 laterally arranged areas (R), the Knopf (or tail-bud). 

 When the embryonic shield proper is formed there 

 appears to be a sort of migration forward and toward 

 the median line of the cells of the Kopf region, which is 



