260 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Figure 98 by the five vertical lines 93-97. Figure 93 shows a section 

 posterior to the blastopore. It passes through one (ms'chy.^ of the small 

 posterior mesenchyme cells, D"^-^, C-^ (cf. Fig. 88), the other lying in the 

 next section posterior to this. The interior of the section is filled with a 

 solid mass of muscle cells, or more properly nerve cells and muscle cells; 

 for it is highly probable that the four most dorsally situated of these 

 cells, which form a group not quite covered in by the ectoderm, are to be- 

 come part of the nerve cord of the tail (cf. Plate XIII. Figs. 99-101, n.). 

 However, they are not distinguishable in histological characters from the 

 more laterally and ventrally situated cells of the section. Cell division 

 has recently occurred in the ectoderm, which plainly is soon to cover in 

 completely the nerve cells in this region of the embryo. The muscle 

 cells have evidently been reduced in size by division since the stage 

 shown in Figures 88 and 89. 



The second section anterior to this is shown in Figure 94. It is the 

 only section of the series which passes thi'ough the blastopore, now 

 reduced almost to a slit. 



The blastopore is bordered on each side dorsally by a large nerve (1) 

 cell, n. (cf. Fig. 90). Ventral to the nerve cells lie the posterior chorda 

 cells, cd., lateral and still ventral to which are muscle cells. The most 

 posterior pair of endoderm cells lies underneath the open blastopore, and 

 a single small mesenchyme cell lies deep down in each half of the section. 



The second section anterior to the blastopore is shown in Figure 95 ; 

 the second section anterior to that, in Figure 96 ; and one situated still 

 two sections farther forward, in Figure 97. 



In Figure 96 the medullary plate is not at all depressed at its centre ; 

 it consists of four large cells closely packed together and columnar in 

 form. In Figure 97 the medullary plate is not even flattened, but con- 

 forms to the evenly rounded contour of the embryo in that region. It 

 consists of six cells sharply distinguished from the cells of the ectoderm 

 in stainability, though the size of the more lateral ones is not materially 

 different from that of the ectoderm cells. The chorda plate has dimin- 

 ished to a breadth of only three cells in Figure 96, and is entirely want- 

 ing in Figure 97, where endoderm cells occupy the space dorsal to the 

 archenteron underneath the medullary plate. The mesenchyme bands 

 cover considerable area in Figure 96, but are reduced to a single cell on 

 each side of the body in Figure 97, from which it is seen that in this 

 region the interior is nearly filled with a solid mass of endoderm. The 

 section represented by Figure 97 lies well toward the anterior end of the 

 embryo, as is indicated by the rapidly diminishing size of the sections. 



