Primitive Streak and Dorsal NotocJiordal Opening. 29 



embryonic area progressed backward. In the present instance, 

 however, the Hps have failed to fuse in the anterior region 

 of the area opaca. Whether the concrescing margins failed to 

 fuse on account of a mechanical obstacle, the presence of yolk, 

 or whether the yolk subsequently protruded through the area 

 of nonfusion, can only be conjectured. This region in this single 

 instance is the only one observed by me that is in any way 

 comparable to Will's uncovered entodermic streak, and in this 

 case the region lies almost entirely outside the area of the shield. 

 It is possible that the streak within the shield may have been 

 formed in this retarded manner. Such a method of development 

 might account for the persisting areas of lateral proliferation and 

 the early differentiation of ectoderm in the axis of the streak. 

 The crescentic dorsal opening of the notochordal canal (Fig. 3) 

 still directs its concavity anteriad; therefore the development of 

 the ectoderm over the streak and the backward turning of the 

 horns of the open blastopore are two entirely independent pro- 

 cesses. Moreover, in the present instance there can be no division 

 of the streak into the " Mittelfeld " and " Randfeld " in the sense 

 in which Will employs these terms, since the middle area has 

 become covered with ectoderm before the separation between 

 ectoderm and entoderm is accomplished laterally. According to 

 Will's theory, this " Mittelfeld " is entodermic, and should be the 

 last region to be covered. 



b. Chclopns insculptus. — Views of the under surface of the em- 

 bryo show (Plate I. Fig. 2') that the notochordal canal has opened 

 ventrally. The concavity of the dorsal crescent-shaped opening 

 (Fig. 2) is still directed anteriad. Only two drawings of sections 

 through the streak region of this embryo will be reproduced, for 

 they will illustrate the histological condition of the whole streak. 

 Figure 47 (Plate IX.) exhibits the third section behind the blasto- 

 pore. The ectoderm is much thickened, and composes the greater 

 portion of the streak. A shallow groove is present along the axis 

 of the streak, and lateral to it there are two depressions, one on 

 either side. These lateral depressions are the first stages in the 

 development of the posteriorly directed horns of the open blasto- 

 pore. These depressions later sink deeper into the streak and 

 divide it into a central elevation — the so called plug of several 

 reptilian embryologists — and two lateral portions. In Figure 47 

 there is no sharp boundary between the columnar ectoderm of the 



