No. 2.] FORMATION OF THE FISH EMBRYO. 459 



be that the body of the Elasmobranch embryo is no more 

 formed by the fusion of two lateral halves than is the body of 

 the Peripatus embryo, in which nearly the whole of the ventral 

 surface is at one time traversed by the long blastopore. 



«' The phenomena we are in both of these cases dealing with 

 is the closure of the blastopore ; and to talk about concrescence 

 and fusion of two halves is merely obscuring the real question 

 and seeking to explain a process of growth by a phrase which 

 has no satisfactory meaning." 



Sedgwick, therefore, holds that the embryo grows backward 

 along two growing lines that are parallel, and end in the tail- 

 knobs. Between these halves an imaginary blastopore is always 

 closing up, so that the blastopore may be spoken of as perfo- 

 rating the whole length of the embryo. Truly, one might turn 

 his own words against him and say that this is " merely obscur- 

 ing the whole question." Whether we accept the theory of con- 

 crescence of the embryo or not, it seems unfair to speak of the 

 conception as " seeking to explain a process of growth by a 

 phrase which has no satisfactory meaning." Even if the 

 phrase is unsatisfactory to Sedgwick, I cannot but believe 

 that, as used by Lereboullet, His, Roux, Hertwig, and Whit- 

 man, the " phrase " has a very clear and definite meaning. 



Hertwig has given a special section of his important paper 

 on "Urmund und Spina bifida" to the Concrescence theory. 

 He says : " Wenn ich daher jetzt zum ersten Mai iiber die Con- 

 crescenztheorie mich ausspreche, so muss ich zuerst hervor- 

 heben, dass ich im Gegensatz zu vielen andern Forschern in 

 der Beobachtung von His, dass sich bei Fischembryonen die 

 Keimwiilste allmahlich von vorn nach hinten zur Formirung 

 der Axenorgane zusammmenlegen, eine sehr wichtige Ent- 

 deckung erblicke. . . . Wenn His versucht, die Concrescenz- 

 theorie^von der Urmundtheorie getrennt zu behandeln, so 

 muss ich hierzu bemerken, dass die erstere nur in Verbindung 

 mit der letzteren uberhaupt verstandlich wird." 



We need not here enter into a discussion of Hertwig's gen- 

 eral conception as to the process of concrescence in the different 

 Vertebrate forms, but we may examine in some detail his con- 

 ception of the process in the Teleost. 



