36 Primitive Streak and Notochordal Canal in Chelonia. 



pore bend backward, extend along the streak, and sink down into 

 it, that the streak becomes so modified that it loses its groove. It 

 is only in rare instances that the groove persists so long as in the 

 case shown in Figure 9. 



Dorsal surface views of the embryos shown in Figures 9, 11, 

 and 13 indicate that these embryos, with the possible exception of 

 Figure 9, belong to a stage later than any embryos previously 

 described in this paper. Figures 5 1 and 53 (Plate X.) are represen- 

 tations of the third sections behind the open blastopores of Figures 

 13 and II, respectively. Neither Figure 53, which is drawn under 

 a high power, nor Figure 51, drawn under a lower magnification, 

 shows any line of demarcation between lateral ectoderm and streak. 

 I am unable to find such a line of separation in any of my sec- 

 tions. Mitsukuri ('93), after carefully re-examining his sections, 

 fails to find such a separation. The ectoderm seems to pass insen- 

 sibly into the cellular condition of the plug. The plug has become 

 a rounded, dull-pointed structure, along whose surface a groove is 

 no longer discernible. The cracks or blastoporic horns which 

 bound this central area laterally contain a few fragments of de- 

 generating cells. Moreover, Figure 51 shows a most interest- 

 ing condition, which I have also observed in two other series of 

 sections. On the outer side of the right horn of the blastopore 

 the ectoderm and entoderm are fused. It is impossible in this 

 region to distinguish a histological condition different from that 

 which exists in the so called plug region. At the angle between 

 the plug and the elevation at the right of it in Figure 5 1 there is 

 a tendency of the superficial layer of cells to become columnar. 

 The mere loss of surface indication of ectoderm is not sufficient 

 therefore to establish the existence of an entodermic plug or yolk 

 plug ; for otherwise in the present case we should have a second 

 plug at one side of the blastopore. 



The opening of the notochordal canal on the ventral floor, as 

 seen in Figure 11', is so far posterior that it persists for only four 

 sections in front of the dorsal lip. The dorsal lip or roof shows 

 evidence of a fusion of ectoderm and entoderm for two sections in 

 front of the anterior termination of the ventral lip. Figure 52 

 (Plate X.) is a representation of the third section in front of the 

 posterior margin of the dorsal lip. A fusion of the halves of the 

 anterior lip can be traced distinctly through six sections. 



Will accounts for the presence of his so called plug and primi- 



