66 



MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



by KUPFFER between the caudal knob (Ck) of the teleost and line of fusion of the 

 blastoderm edges behind the embryo of the shark is in principle true, though not 

 quite exact. The caudal knob of the teleost represents, rather, the embryon'c tail- 

 end of the shark, enclosing the neurenteric canal. (This is the view maintained 

 by SCHWARTZ '89, though I did not know it when the foregoing words were written. 

 Concerning KOPSCH, see below.) The posterior line of fusion (Bl) has, strictly speak- 

 ing, no counterpart in the teleost, inasmuch as the embryo retains its connection 

 with the blastoderm margin. 



H. VIRCHOW ('95) proposes a view of the teleost embryo similar in some respects 

 to those of KUPFFER and myself. He considers the caudal knob to result from a 

 process of folding similar to that in the elasmobranch. In the latter, he says, all 

 three of the germ layers are folded off and the tail projects freely, while in the trout 

 the folding process affects merely the ento- and mesoderm, the ectoderm taking no 



FIGURE 29. 



Schematic figures of A, early elasmobranch (after Balfour), and B, teleost embryo illustrating the author's view of 

 the formation of the teleost embryo. Mg, medullary groove ; Ne, neurenteric canal ; Bl, line of fusion of edges of 

 blastoderm behind the latter ; Yk, yolk. 



part in the process. This ventral folding off of the entoderm results in the forma- 

 tion of Kupffer's Vesicle which is thus, in its origin, like any other part of the gut. 

 But as VIRCHOW does not recognize the presence of the " prostoma," nor the part 

 played by concrescence, his account is incomplete. He attributes a similar view to 

 OELLACHER, though I do not recall the latter's statement. The views of KOPSCH 

 and JABLONOWSKI will be considered below. 



If the theory I have advocated is correct, it is evident that the development of 

 the teleost egg differs far less from that of the other meroblastic vertebrate eggs than 

 has usually been held. In all of these, it seems probable that the originally simple 

 blastopore, consisting of the whole exposed surface of the yolk, has been separated 

 by a process of concrescence into an anterior, embryonic portion and a posterior, non- 

 embryonic portion. For the bird's egg, the case has been convincingly presented by 

 DUVAL ('84). The primitive streak, although appearing to originate at some distance 



