Primitive Streak and Dorsal Notochordal Opening. 19 



the blastoporic canal has not yet attained its full normal transverse 

 extent, and certainly its antero-posterior length falls short of the 

 normal condition. Mitsukuri ('93, p. 248) found that the invagi- 

 nation cavity at an early stage is a " squarish pit rather elongated 

 in the antero-posterior diameter." Kupffer ('82, Taf VIII. Figs. 7, 

 8, 9) illustrates stages found in blastoderms of the chick incubated 

 between nine and twelve hours, in which the prostoma possesses 

 an anterior linear extension. A posterior prolongation also is 

 shown in his Figures 8 and 9. This condition, which sometimes 

 appears in the chick, is comparable to that found in the Chelydra 

 embryo in the present instance. The transverse extent of the 

 Chelydra prostoma is, however, much more limited than that of 

 Kupffer's chick, while the extension of the groove anterior to the 

 prostoma is greater in the present instance than in the chick. In 

 both cases the groove continues anterior to the region of fusion 

 between ectoderm and entoderm. The seventh to ninth sections 

 of this Chelydra embryo show a condition in which ectoderm and 

 entoderm are separated from each other, and in which both are 

 grooved in the axial line. A drawing of section nine (Fig. 3 1) 

 illustrates this condition. It is anomalous that the ectoderm in 

 this embryo seems to contain as many yolk globules as the ento- 

 derm, or even more. From section ten onward, we find the ecto- 

 dermal halves of embryo a to be separated in the mid line. The 

 drawing of section thirteen (Plate V. Fig. 24) has been less highly 

 magnified than the two sections already described, in order that it 

 might include that portion of individual yS which appears in this 

 region. Figure 24 illustrates a condition which occurs with but 

 little variation for twenty-two sections. The ectoderm of the two 

 halves of embryo a no longer meets in the mid dorsal line. The 

 entoderm, however, is continuous across the median plane, some- 

 times, as in Figure 24, with little or no indication of an axial 

 thickening. That portion of Figure 24 included between the let- 

 ers /3' and yS" will, upon reference to the surface view (Plate II. 

 Fig. 5), be seen to belong to the antero-dextral portion of indi- 

 vidual /S. Individual /8 is growing in an anterior direction, and, 

 being the more advanced oi" the two and the more normally de- 

 veloped, it crowds a by pushing itself anteriad, either over a, as at 

 the point /3" (Fig. 24), or underneath a (Plate V. Figs. 25, 26, and 

 Plate VI. Fig. 27). As soon as the line of continuity between the 

 ectoderm of the two individuals is broken, /3 slips underneath a 



