SUMNER: KUPFFER'S VESICLE 65 



of the yolk has been covered, and Kupffer's Vesicle appears somewhat later ; in Scor- 

 pxna, the relative time of formation of both of the structures is still later, at least 

 four-fifths of the yolk being covered ; while in Mursena f the vesicle does not ap- 

 pear to view until the blastopore is nearly closed, and the caudal end of the embryo 

 is conspicuously bifid up to the time when the blastopore is very small. Of course 

 this line of argument is open to the reply that the time of appearance of both these 

 structures is conditioned by the general rate of development of the embryo in any 

 particular case and that the coincidence stated does not prove any necessary relation- 

 ship between the two. 



As might be expected, an indentation of the posterior end of the embryo 

 has been already noted by several investigators. AGASSIZ and WHITMAN ('84) once 

 or twice observed this condition in living pelagic eggs and regarded it as strong evi- 

 dence for the formation of the embryo by concrescence. HENNEGUY ('88) noted its 



FIGURE 27. FIGURE 28. 



The posterior end of embryo of Marietta? just prior to Oblique section of preceding embryo, 

 closure of blastopore. Yp, yolk-plug ; Pr, " prostoma " ; 

 X-Y, plane of section shown in FIGURE 28. 



existence in a single egg and from this concluded that concrescence occurred in the 

 fish embryo in a very limited region, i. e., enough to form the caudal knob, although 

 not giving to this process any such interpretation as we have had under discussion. 

 M'INTOSH and PRINCE ('90) speak of a " terminal bay " in certain pelagic eggs. 

 EYCLESHYMER ('95) notes that this indented condition has been already observed in 

 Amiurus by Miss O'GRADY, of Vassar. JABLONOWSKI ('98) describes the artificial 

 production of a similar condition in the Salmonidse, by the use of salt solution. 



Reference might also be made to the Mesodidymi arid Hemididymi of various 

 authors, although I do not consider it necessary to discuss them here. 



Thus the prostoma of the teleost, like the neurenteric canal of the shark, repre- 

 sents a specialized portion of the blastopore which has become detached from the 

 remainder by a process of concrescence or union of the blastopore lips. Figure 29, 

 A and B, illustrate this comparison. It will be seen that the homology suggested 



