BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
stage as is represented in Figure 80, four sections have been solected to 
make more clear the relations of the fundaments of the various orpans. 
(See Plate XII. Figs. 84-87.) Figure 84 represents a section just 
behind the blastopore (compare with it Plate XI. Fig. 73); in it the 
ectoderm is seen to have slightly overgrown from behind the most pos- 
terior musele cells. (Compare Fig. 79, Plate XIL) Only one of the 
pair of small posterior mesenchyme cells (D'5, C75) appears in the 
section; the other lies in the section just posterior to this. 
Mitosis is again setting in among the endoderm cells, as is shown by 
the spindle in d7%; the next section anterior to this likewise shows 
spindles in the endoderm cells that are cut, c"?! and d, The spindles 
in each case (^7 and 7%) are directed longitudinally, and in such a 
manner that. the eight resulting cells will all lie in a single slightly con- 
cave layer. The consequence of these divisions will be a considerable 
elongation of the double row of endoderm cells at the posterior end of 
the embryo. 
It is worthy of note, though not shown in this serios of sections, that 
at this division, as in the preceding and in subsequent ones, the spindles 
of the endoderm cells do not lie in the longest axis of the cells, which is 
the vertical. 
Figure 85 shows a section through the still open blastopore at its 
posterior margin. A comparison of this figure with Figures 72, 74, and 
75 (Plate XL) shows that the ectoderm has grown rapidly in superficial 
extent through cell multiplication, and shoved the neuro-muscular cells 
0%, D inward to a position overlying their sister cells, O94, DM, 
The small mesenchyme cell, 09-1 (cf. Plate XI. Fig. 74, D%15), is in pro- 
cess of division, following the lead of its largo sister cell, O°"! (of, Wigs: 
74, D5, and 84, D2, 79. The mate of 0810, viz, .D*15 has already 
divided. One of its daughter cells is seen in this section (DM Pig. SU, 
the other lies in the next posterior section. The large mesenchyme cells, 
pes Er EUN SRM 85), are in mitosis (cf. Plate XI. Fig. 83). 
The section represented in Figure 86 encounters the blastopore farther 
forward than the one last described (Fig. 85), in its broader portion (cf. 
Plate XI. Figs. 72, 76, and 77). Here, too, the muscle cells have been 
crowded inward and partially invavinated ; C935 and DI overlie their 
sister cells, 091 and DI, respectively. Of the posterior chorda cells 
only c*2 and q8 appear in this section. Their moro laterally placed 
sister cells, c5:2 and d8L lie in the next posterior section (not figured), 
and at a slightly higher level (cf. Fig. 81, Plate XI). 
Figure 87 (Plate XIL) represents tho first section anterior to the 
