258 bulletin: museum of compaeative zoology. 



(Plate XII.) mesenchyme cells, three on each side. Two of them are 

 undoubtedly descendants of the mesenchyme cells C^'^^, D^-^^, shown in 

 Plate XI. Figs. 73 and 74, and represented in Plate XI. Fig. 83, and 

 Plate XII. Fig. 84, by the cells C'-''^ (7^-8«, B^-"^, and D'-^'. Their nuclear 

 condition shows that they have arisen from a recent division. Dorsal to 

 the groups of cells already mentioned are seen in Figure 89 miiscle cells 

 extending up in a solid mass to the dorsal surface of the embryo. In 

 the mid-dorsal surface of the section is a pair of cells, probably nerve 

 cells, between which at an earlier stage lay the open blastopore. The 

 periphery of the section is elsewhere bounded by ectoderm. 



Figure 90 (Plate XII.) represents the second section anterior to that 

 shown in Figure 89. It passes through the widest portion of the blasto- 

 pore. The only other section of the series which passes through the 

 blastopore is the next preceding one, in which the blastoporic opening 

 is extremely narrow, in fact, scarcely more than a slit. Figure 91 

 shows the first section anterior to the blastopore. In it we see a plate 

 of seven cells (cd.) belonging to the anterior chorda fundament and form- 

 ing the roof of the archenteron (cf. Fig. 81). In Figure 90 we find the 

 posterior chorda cells (cd.) lateral to the blastopore (cf Plate XI. Fig. 81, 

 c^--^ and c^-^^). Ventral to the chorda cells in Figures 90 and 91 are 

 the mesenchyme cells descended from c^-^', c^-^**, d^-^% and d^-^* (cf. 

 Plate XI. Figs. 82 and 83). 



Dorsal to the chorda cells in Figure 91 are four cells unquestionably 

 nervous, the two lateral ones being large and in mitosis, the other two 

 small, evidently produced by recent divisions. In the next anterior 

 section (not figured) the two lateral mitotic nerve cells again appear ; 

 completely filling the space between them are four small nerve cells 

 similar to the two seen in Figure 91. A medullary groove is thus clearly 

 formed anterior to the blastopore, and the four cells dorsal to the chorda 

 fundament in Figure 91 evidently are only lateral backward prolonga- 

 tions of the medullary plate. The two large cells at the margins of the 

 blastopore in Figure 90 are probably C^-^^ and Z)^-^^ (cf. Fig. 86) ; their 

 deeper lying sister cells C^'^^ and Z)^-^® have been carried into the more 

 posterior sections by the crowding backward of the chorda cells and the 

 elongation of the embryo. 



Figure 92 (Plate XII.) represents the third section anterior to the 

 one shown in Figure 91. The medullary plate and chorda are here 

 represented each by four cells. The mesenchyme cells visible on each 

 side of the archenteron are A^-^, B^-^\ A^-^\ and £^.^' (cf. Plate XI. 

 Fig. 82). The cells ^^•-^ B'-^\ A^-^% and B^-^^ (cf. Fig. 82) lie in the 



