30 Primitive Streak and Notochordal Canal in Che Ionia. 



lateral areas and the ectoderm of the central area. Six sections 

 behind the open blastopore (Plate IX. Fig. 48) the ectoderm over 

 the surface of the streak becomes completely differentiated from 

 the underlying cells. Here and there, however, it is fused for the 

 space of a few cells with the entoderm. In the lateral regions it is 

 possible to distinguish the splanchnic and somatic layers of the 

 mesoderm ; but in the centre of the streak entoderm and meso- 

 derm are indistinguishable. 



This and the preceding examples are sufficient to show that the 

 primitive groove exists along the streak before the turning back- 

 ward of the margins of the open blastopore. Therefore, if the 

 backward growth of the blastopore should produce a groove on 

 the streak, this groove must be a structure quite different from 

 the primitive groove. 



Although it is impossible for me in the two instances of an 

 elongated streak previously described, to connect these streaks 

 with the edge of the blastoderm, as Whitman ('83) has done in the 

 case of an abnormal chick, yet it seems impossible to account for 

 the presence of a streak over so great a portion of the area opaca, 

 unless we grant that the edges of the germ ring have continued 

 to unite for a much longer distance than usually occurs in birds 

 and reptiles. Therefore my two embryos seem to me to form an 

 intermediate stage between the normal streak of reptiles and that 

 described by Balfour for Elasmobranchs, and by Whitman for his 

 abnormal chick. 



I have little to add to the question whether the whole germ 

 ring represents the blastopore (Rauber, 'yy, '80; His, '78), and the 

 embryonic axis only a portion of the blastopore, or whether only 

 a portion of the germ ring represents the gastrula mouth (Hertwig, 

 '92, '93; Duval, '78, '84; Minot, '92; and Cunningham, '86). If 

 the blastopore has experienced a hernia, as Cunningham suggests, 

 concrescence of the non-blastoporic germ ring could continue 

 nevertheless. The condition of the separated ectodermal borders 

 along the streak in the region of the area opaca, as seen in Fig- 

 ures 41 and 42, would seem to yield evidence for the His-Rauber 

 theory, since in this region the ectoderm is decidedly infolded. 

 At the prostoma marginale the ectoderm goes over into the ento- 

 derm (Rauber, '83, p. 165). But this infolding and proliferation, 

 as seen in Figures 41 and 42, may have a mechanical cause only. 

 It is difficult, however, in the case of Figure 42, to conceive of 



