42 Primitive Streak and NotocJiordal Canal in Chelonia. 



Taf. 35, Figs. iS'^-iS"; and my own Figures i8 and 19, Plate IV., 

 and Fig. 48, Plate IX.) Whether there is on the streak an actual 

 groove or not, one is forced to conceive of the existence of 

 a potential archenteron in this region. VVlien a primitive groove 

 does exist, there is no difficulty in conceiving of an archenteron, 

 — one, however, that is so compressed that it exists only as 

 a fold or closed space between cell walls. Such a modified 

 archenteron is seen to exist in Figure 23 (Plate V.). For the 

 sake of emphasizing my view I have in Diagrams III.-VIII, 

 drawn this archenteron as though it were open at the base of 

 the groove. I have represented potential notochord by dots ; the 

 remaining portions of the several germ layers are so lettered (see 

 Abbreviations, p. 54) as to be readily understood. A potential 

 archenteron is represented as surrounded by the entoderm. 



As we pass anteriad along the streak, we come (Diagram VII.) 

 into the region of the extreme posterior end of the backward- 

 turned horns of the crescent, where the streak is beginning to sink 

 below the level of the embryonic surface. The central portion of 

 the streak is little if at all affected by this sinking; it often pos- 

 sesses along its axis a primitive groove. Mchnert's ('92) Figure 

 25 (Taf. XIX.), and my own Figures 47 (Plate IX.) and 56 (Plate 

 XI.) illustrate this condition. 



If we pass on anteriad until we come near the posterior lip of 

 the open portion of the blastopore, we meet the condition seen in 

 Diagram VI. The two lateral depressions have sunk very deep, 

 while the axis of the streak is little affected, and possesses ofttimes 

 a primitive groove. Such a condition is represented by Mitsukuri 

 and Ishikawa, 'm. Fig. 18, Plate IV.; by Will, '93, Fig. 24^ Taf. 

 36; and by my own Fig. 49 (Plate X.) and Figs. 55 and 58 (Plate 

 XL). 



At a little later stage in the section from this region, or at this 

 stage in sections a short distance in front of it, this whole central 

 area (Diagram V.) is seen to have sunk some distance below the 

 embryonic level, and to have changed to the form of a blunt cone. 

 Mitsukuri's ('86) Figure 9 (Plate III.), and my Figures 50 and 51 

 (Plate X.), represent this condition. At the stage in which this 

 plug area has been transformed into a cone all traces of a primitive 

 groove have been obliterated. In earlier stages, however, the 

 groove extends into this region. 



At a point one or two sections farther forward (Diagram IV.) 



