264 bulletin: museum of comparative zoology. 



the caudal endoderm cells, i. e. are resolved into mesenchyme at a later 

 stage. 



Figure 102 (Plate XIII.) represents a section through what probably 

 was the region of the blastopore. In it is seen the most posterior pair 

 of endoderm cells (cf. Plate XII. Pig. 94) . Since the stage last exam- 

 ined (Plate XII. Figs. 93-97) the chorda cells have closed together into 

 a single plate in this region, and the chorda fundament has grown farther 

 back in the embryo. The nerve cells which lay at each side of the 

 blastopore (Plate XII. Fig. 94) have also met in the median plane to 

 form a single plate, which is now closing into a canal. A real canal is 

 never formed posterior to the blastopore, though the nerve cells in that 

 region potentially form one. 



Figure 103 (Plate XIII.) represents the second section anterior to the 

 one shown in Figure 102 ; Figure 104, the second anterior to that; and 

 Figure 105, the fourth anterior to that. It will be seen that the muscle 

 cells which in the series last examined (Figs. 93-97) were aggregated 

 chiefly behind the blastopore, have now extended themselves not only 

 posterior, but also anterior, to the blastopore. They extend as far for- 

 ward as the next section in front of the one represented by Figure 103 

 i. e. through three sections anterior to the blastopore. They have 

 pushed before them the mesenchyme, which in this series first appears 

 in the section shown in Figure 103. The chorda fundament has mean- 

 while moved toward the posterior end of the embryo. It now extends 

 two sections behind the blastopore and overlies the small postei'ior me- 

 senchyme cells (Fig. 101, cf. Plate XII. Fig. 93). Accompanying the 

 changes just mentioned, has come a diminution of the diameter of the em- 

 bryo at its posterior end, which is already elongating to form the tail 

 region. 



The mesenchyme extends forward of the section shown in Figure 103 

 through six sections. The medullary plate extends forward two or three 

 sections farther still. The endoderm consists of a double row of cells 

 extending forward underneath the chorda as far as the section seen in 

 Figure 104, in which four endoderm cells are found; the arrangement 

 there shown has been derived from that shown in Plate XII. Fig. 96, 

 and still earlier in Plate XII. Fig. 91, by the meeting in the median 

 plane underneath the chorda of the more laterally placed endoderm cells. 

 Later, these four cells, or their descendants, will move apart so as to en- 

 close between them the lumen of the posterior portion of the digestive 

 tract. Anterior to the section shown in Figure 104 the endoderm 

 rapidly increases in amount, while the chorda and mesenchyme diminish. 



