Primitive Streak and Dorsal Notochordal Opening. 27 



this is the last section of the series which has been preserved, for 

 during the process of embedding the remaining portion of the 

 blastoderm broke away. An explanation of this thickened line of 

 entoderm, continuous with the primitive streak, and extending 

 backward through the greater portion at least of the area opaca, 

 will be suggested during the description of the next older shield. 



The notochordal canal of this next older embryo (Plate I. 

 Figs. 3, 3', and Plate VIII. Fig. 35) has already been described; 

 therefore we will immediately direct our attention to the study of 

 a series of cross sections of its primitive streak. Figure 36 (Plate 

 VIII.) represents the condition found in the fifth section behind 

 the blastoporic opening. The histological conditions of this section 

 and the four anterior to it are almost identical. The positions 

 of this and the following sections are shown on Figure 44 (Plate 

 VIII.). Mesoderm and entoderm are continuous along the whole 

 breadth of the streak. The ectoderm is continuous with the lower 

 layer at two points, one on either side of a central region, in which 

 the ectoderm is completely separated from the underlying cells. 

 In the ninth section (Plate VIII. Fig. 37) posterior to the open 

 blastopore, this central region is somewhat arched. The ectoderm 

 is still proliferating cells into the lower layer at the right of this 

 central arch. In Figure 38 the central area has become still more 

 elevated, and the two lateral areas, which were centres of prolifera- 

 tion in Figure 36, have now become two compact clumps of cells, 

 which are indissolubly fused with the entoderm below. Upon 

 reference to surface views (Plate I. Figs. 3 and 3') a central eleva- 

 tion is seen to continue backward along the region of the streak. 

 The portions at the two sides of the streak seem to be composed 

 each of two terraces. Figure 38 passes through the posterior end 

 of the anterior (or inner) terrace. Figure 39 passes through the 

 posterior terrace twenty-three sections behind that of Figure 38. 

 In this section (P'ig. 39) the streak has become narrower and 

 much flatter. On account of the presence of a layer of hardened 

 yolk, which lies upon the dorsal surface of the streak at this point, 

 it is impossible to make out the exact condition of the streak from 

 surface views. The next posterior section, of which Figure 40 is 

 a representation, passes very near the posterior margin of the 

 shield. In this section the ectoderm has parted in the median 

 plane, so that the yolk has streamed out through the gap and 

 spread over the surface of the streak. In the thickened central 



