FORMATION OF NOTOCHORD IN AMPHIBIA. 29 I 



of the section of the earlier embryo shown in Fig. 2. The 

 notochord is triangular in shape and is closely connected with the 

 layer of cells forming the mid-dorsal wall of the archenteron. 

 The portion of the dorsal plate directly under the notochord is 

 cut off on either side from the rest of the layer, and to it one 

 can, perhaps, fitly apply the term " chorda-endoderm," since it 

 is destined to become a part of the notochord. At this stage of 

 development, the dorsal plate is much narrower in the posterior 

 region of the embryo than it was before the medullary folds 

 formed (Fig. 2), and it is again found to be directly connected 

 with the lower layer of mesoderm and not with the yolk cells 

 forming the lateral walls of the archenteron. 



o 



In Fig. 4, a portion of a section slightly anterior to that 

 shown in Fig. 3, the chorda-endoderm is seen to be the only 

 portion of the dorsal plate bordering the archenteron. The 

 other cells of the dorsal plate have united with the mesoderm, 

 and can only be distinguished from it on account of their posi- 

 tion and the fact fehat they contain somewhat more pigment. 

 The entire dorsal wall of the archenteron, excepting the part 

 formed by the chorda-endoderm, is here composed of large, 

 rounded yolk cells which are evidently growing up from both 

 sides, and thus shutting off all of the cells of the dorsal plate 

 from bordering the archenteric cavity. More anteriorly, as shown 

 in Fig. 5, the yolk cells of the upper wall of the archenteron are 

 still closer together in the middle lines. In this part of the em- 

 bryo the cells of the chorda-endoderm no longer form a nearly 

 straight line at the lower edge of the notochord, but they have 

 become an integral part of it, and most of their pigment is 

 collected in the form of a pronounced ring around the center 

 of the notochord. 



Near the middle of the embryo (Fig. 6), the yolk cells have 

 almost met under the notochord, which is smaller and more 

 rounded than it is in the posterior part of the embryo. A section 

 more anteriorly still (Fig. 7) shows that the yolk cells from the 

 two sides of the archenteron have come together in the middle 

 line under the notochord. As a result, the dorsal wall of the 

 archenteron is composed entirely of a single layer of large yolk 

 cells, and the cylindrical notochord above it is cut off entirely 



