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MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



The emarginate condition is also well shown in the transparent living egg of 

 Scorpsena. It is true that figure 25 shows a shallow bay rather than sharp nick, 

 but the conspicuous shingle-like overlapping of the cells along the whole posterior 

 margin speaks strongly for a process of concrescence. This appearance of the blas- 

 toderm margin recalls the descriptions of RYDER ('86) for Elacate and of LOCY ('95) 

 for Squalus, and it seems quite possible that the structures which were by those 

 writers given a segmental value find their true explanation here. In figure 26 

 the emarginate condition has for some time ceased to exist and Kupffer's Vesicle 

 has come into view at a considerable distance from the margin. Behind this the 

 caudal knob projects into the yolk blastopore. 



In the egg of Mursena f which, as above described, exhibits the prostoma in its 

 least modified form, there is also the strongest evidence of this view of its formation. 



FIGURE 25. 



FIGURE 26. 



Pr' 



Living egg of Scorpsena. Pr, emarginate region at pos- 

 terior end (not often as deeply concave as in figure). Ar- 

 rangement of cells on either side strongly suggests con- 

 crescence. 



Kv 



Scorpiena embryo after caudal knob and Kupffer's Vesi- 

 cle have appeared to view. 



Figure 27 shows a deep bay extending forward from the blastopore into the pos- 

 terior end of the embryo. It is evident from transparent view that the yolk is not, 

 however, exposed in this bay as in the rest of the blastopore (compare with condi- 

 tion in Noturus, shown in figure 23). The section (figure 28) verifies this opin- 

 ion. It is at once seen that at this point the invagi nation to form the hypoblast (as 

 above discussed) is occurring. 



In this connection it is significant to note that* there seems to be a certain rela- 

 tion between the time of appearance of the caudal knob and the age at which Kupf- 

 fer's Vesicle is formed. In the embryo of the trout, the caudal knob appears at a 

 period when the blastoderm occupies but a small part of the upper hemisphere of 

 the egg. The definitive Kupffer's Vesicle is formed some time before the equator 

 has been passed. In Noturus, the caudal knob is not developed until about one-half 



