No. I.] GAR-PIKE AND STURGEON. 45 



surrounding and indistinct zone of dusky color is mainly 

 mesoblastic, while outermost the dark-colored parietal region 

 indicates the general boundary of the fore-gut : in its fore- 

 most margin is the anlage of the heart. Pronephric ducts 

 appear at the sides of the neural axis, terminating in a some- 

 what obscure way in the neighborhood of the hind-brain. 

 As yet traces of neither gill slits nor auditory vesicles have 

 appeared. 



The inner germ layer m.3.y next be considered. In the stage 

 of PI. IV, Fig. 64, the entoderm may be traced from within the 

 rim of the blastopore, lining its margins, continuous with the 

 cells of the surface of the yolk mass. Upon the closure of the 

 blastopore, Figs. 66, 6"], 70, the entoderm of the outer 

 wall of the coelenteron is a layer of cells distinctly separate 

 from outer layers : at its lateral margins it gradually becomes 

 continuous with the cell layer of the yolk which constitutes 

 accordingly the inner wall of the gut. The yolk is therefore 

 to be interpreted as identical in its relations with that of Elas- 

 mobranch, Teleost, or urodele, and is not to be compared to that 

 of Ichthyophis, as suggested by Ryder, Ref. 22. At this stage 

 the extent of the gut is to be seen in the sagittal and cross 

 sections above referred to. The outer limit of the parietal 

 zone of Figs. 50, 5 r marks in general the boundary of the 

 gut. The notochord arises in the normal manner, a rod-like 

 thickening of the hypoblast in the line of the embryo's 

 axis. Beginning at the rim of the blastopore, thick and wide, 

 as it extends forward it diminishes in size. Fig. 70, and is 

 finally indistinguishable in the region of the hind-brain. The 

 writer can find nothing in the mode of origin of the noto- 

 chord to suggest the mesoblastic derivation maintained by 

 Salensky. 



The mesoblast in Acipenser was regarded by Salensky as 

 derived from the entoderm in as early a stage as that figured 

 in PI. IV, Fig. 62. Here the entire non-entodermic portion of 

 the rim of the blastopore seems to be identified by the Russian 

 author as 'mesoderm,' the entoderm extending around the yolk 

 mass but reaching no farther than the ring-like end of the coelen- 

 teron. This ' mesoderm ' was accordingly described as growing 



