258 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(Plate XII.) mesenchyme cells, three on each side. Two of them are 
undoubtedly descendants of the mesenchyme cells C55, DB, shown in 
Plate XI. Figs. 73 and 74, and represented in Plate XI. Fig. 83, and 
Plate XII. Fig. 84, by the cells 0, C2) D? 9, and 799). 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 muscle 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 blastoporie 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 (ed.) 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 (ed.) lateral to the blastopore (cf. Plate XI. Fig. 81, 
ce and c*?5, Ventral to the chorda cells in Figures 90 and 91 are 
the mesenchyme cells descended from 6939, (9%, ¿82% and q*7^ (ef. 
Plate XI. Figs. 82 and 83). 
Dorsal to the chorda cells in Figure 91 are four cells unquestionably 
nervous, the two lateral ones being larpe 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 C945 and DI (cf. Fig. 86) ; their 
deeper lying sister cells O9: and 945 have been carried into the moro 
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 49-99, IAEA AO SSH (Ue ITE De ASIN 
Meal) „ung gols A A OI Cor MS) lo ih the 
