No. 2.] FORMATION OF THE FISH EMBRYO. 457 



overgrow the yolk and to a differentiation of the retarded lips 

 into the halves of the embryo. 



If, therefore, in the frog, spina bifida is due to the imperfect 

 concrescence in the two sides of the blastopore, are we to con- 

 clude that, when we find spina bifida in the fish, the germ- 

 ring also forms the embryo by a process of apposition ? I think 

 not. It is useless, however, to discuss the matter without 

 additional evidence. I cannot let the question pass, however, 

 without this statement : The axial concentration of the material 

 to form the embryo would supply the basis for an explanation 

 of a doubling in the fish. Again, it is a most significant fact 

 that while in the frog embryo with spina bifida the yolk-cells 

 are always exposed, yet we find in the fish, in all cases figured, 

 a bridge of ectoderm connecting the separated sides of the 

 embryo. We must not, moreover, forget that the fish and frog 

 are formed on different parts of the ^gg ; and that finally, in 

 the development of the frog we do not find any large amount 

 of tissue laid down in the head region before the closing in of 

 the blastopore begins. These facts go far towards freeing 

 one from the supposed necessity of finding exactly the same 

 explanation for the two cases of spina bifida. 



His (6) has recently (1891) reiterated his belief in the con- 

 crescence theory. He says the length of the head of the 

 embryo-fish remains the same when the embryo is 1.2 mm. and 

 when it is 4 to 5 mm. Further, the early blocks of mesoderm 

 retain the same relative position throughout the development. 

 "Somit findet das Korperwachstum nicht durch eine innere 

 Dehnung der ersten Anlage statt, sondern dadurch dass den 

 zuerst vorhandenen Teilen von hinten her neue sich anfugen. 

 Das Wachstum geschieht mit anderen Worten nicht durch 

 Intussusception, sondern durch Apposition." 



It is impossible to think. His adds, that a sudden increase 

 of new material takes place during overgrowth, for he has 

 shown (in 1878) that the amount of material at the beginning 

 and end during the overgrowth is the same in amount, counting 

 both germ-ring and embryo. 



His speaks in this paper of a large amount of lateral com- 

 pression taking place in the early embryo, since the breadth 



