256 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



stage as is represented in Figure 80, four sections have been selected to 

 make more clear the relations of the fundaments of the various organs. 

 (See Plate XII. Figs. 84-87.) Figm-e 84 represents a section just 

 behind the blastopore (compare with it Plate XL Fig. 73) ; in it the 

 ectoderm is seen to have slightly overgrown from behind the most pos- 

 terior muscle cells. (Compare Fig. 79, Plate XII.) Only one of the 

 pair of small posterior mesenchyme cells {D^-^, C*'-^) 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 d?-^^; the next section anterior to this likewise shows 

 spindles in the endoderm cells that are cut, c'^"" and rf"". The spindles 

 in each case Q'^^ and ''•") are directed lougitudinall}', 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 he 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 series 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 w^ith Figures 72, 74, and 

 75 (Plate XI.) shows that the ectoderm has grown rapidly in superficial 

 extent through cell multiplication, and shoved the neuro-muscular cells 

 ^9.13^ ^9.13^ inward to a position overlying their sister cells, C^'", D^-^^. 

 The small mesenchyme cell, C^-^^ (cf. Plate XI. Fig. 74, Z)^-^^), is in pro- 

 cess of division, following the lead of its large sister cell, C^'^^ (cf. Figs. 

 74, Z)«-'^ and 84, D^-~\ Z)^-^^). The mate of C^'^\ viz. D'-^\ has already 

 divided. One of its daughter cells is seen in this section (Z)^-'^, Fig. 85), 

 the other lies in the next posterior section. The large mesenchyme cells, 

 c»-23, rf8.23 (Y\^. 85), are in mitosis (cf. Plate XI. Fig. 83). 



The section repi'esented 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 invaginated ; (7^-i5 ^nd D^-^^ ovei'lie their 

 sister cells, (7^-i® and D^'^^, respectively. Of the posterior chorda cells 

 only c^'^^ and d^--^ appear in this section. Their more laterally placed 

 sister cells, c^'^^ and t?®-^^, lie in the next posterior section (not figured), 

 and at a slightly higher level (cf. Fig. 81, Plate XL). 



Figure 87 (Plate XII.) represents the first section anterior to the 



