No. I.] GAR-PIKE AND STURGEON. 25 



SO exceedingly flattened that it can be discerned only with 

 difficulty. The floor of the vascular enlargement is the region 

 of especial interest ; it is already covered with spherical mesen- 

 chyme elements which are seen in process of being budded off 

 from the yolk ; they have certainly this origin, although the 

 writer believes that they may also be derived from the marginal 

 cells of both the upper and lower layers : the conditions how- 

 ever that are here presented appear strikingly similar to those 

 figured by Riickert in Selachian. The fate of these vascular 

 elements will later be referred to. 



Early Embryo. 



The stage in which the outward shape of the embryo begins 

 to appear is figured in PI. I, Fig. 19. Its axis is almost a 

 straight one, and the slender embryo is sharply constricted off 

 with head and tail eminences especially prominent above the 

 surface of the rounded yolk. The head eminence is the 

 embryo's widest part, its outline is bluntly lanceolate, its 

 dorsal surface is flat, but shows the presence of a faintly 

 marked cord of cells in its median line. The tail eminence is 

 less prominent ; it is well rounded, highest in its posterior 

 part. The middle region of the embryo remains as yet at the 

 ftg^ surface. A slight rounding in of the surface at the sides 

 of the embryo is the first indication of a parietal zone. The 

 following structures take their definite origin during this stage : 

 notochord, primitive segments, central nervous system and 

 blood vessels. Of the pronephric duct early traces are found 

 but its more definite appearance is at a later stage. The noto- 

 chord takes its origin from the hypoblast in a manner very 

 similar to that of the Teleost ; by the time the entoderm has 

 spread under the embryo its thickening is noticeable in the axial 

 line ; the ridge that is thus formed is more prominent near the 

 hinder body region and it is here that the chord is first seen 

 separate from the entoderm. Its appearance at this stage is 

 shown in PI. II, Fig. 33, c. In the embryo described four 

 primitive segments are present ; the mode of their formation 

 is to a degree shark-like inasmuch as the visceral and parietal 



