THE FORMATION OF THE FISH EMBRYO. 



T. H. MORGAN. 



TABLE OF CONTENTS. 



PAGE 



Normal Development 419 



„ „ ( Removal of Blastomeres 4^4 



Experimental Observations : \ ^-^^ 



'Cutting the Germ-ring 440 



Abnormal Development 442 



Historical and Critical Review 446 



The conception of the formation of the vertebrate embryo 

 by a process of concrescence has gained many adherents since 

 its formulation for the fish by Lereboullet in 1863, and by His 

 in 1874. 



Nevertheless, the theory has, from the start, met with no 

 little opposition from other workers who have covered a wide 

 field of investigation. 



The solution of the problem in the teleost — the form that 

 gave the starting-point for the theory — seems by no means 

 impracticable, and to involve only a very careful study, both 

 quantitative and qualitative, of the early stages of development. 

 Particularly does the solution seem to be within reach of the 

 experimental method. 



During the summer of 1893 I tried the application of this 

 method to the problem. The germ-ring, from which, on the 

 theory of concrescence, the two halves of the embryo arise 

 by a process of apposition, was cut off on one side from its 

 connection with the embryo. Nevertheless, both sides of the 

 embryo that resulted were superficially alike and equal. The 

 obvious conclusion seemed to follow that the germ-ring took 

 no extensive part in the formation of the body of the fish. 



The conclusion reached is not beyond legitimate criticism, 

 for the question remains unanswered, whether under such con- 



