250 bulletin: museum of comparative zoology. 



have taken place since the stage shown in Plate XL Fig. 71, has come 

 about chiefly by an invagination, independent of cell division on the 

 aboral surface, which has carried inward the endoderm and mesenchyme 

 cells, has left at the margin of the wide-open blastopore the anterior 

 chorda cells and the muscle cells, and has brought plainly into view, 

 outside the neuro-muscular band, another row of cells from the ecto- 

 dermal surface extending round the entire margin of the embryo. A 

 certain number of cells at the anterior end of this new ring is destined 

 to serve the same purpose as the anterior segment of the neuro-muscular 

 band ; this fact is indicated in the figure by stippling. 



A clearer idea of the changes just sketched in outline may perhaps be 

 had from an examination of cross sections. In Figures 73-77 are rep- 

 resented five sections through the region of the blastopore of an embryo 

 a little more advanced than the one shown in Figure 72. The approxi- 

 mate position of the sections in the embryo is indicated by horizontal 

 liues at the margin of Figure 72. In the endoderm cells the long 

 deferred division leading to the seventh generation has at last been 

 accomplished (Figs. 73-77). The endoderm cells accordingly number 

 twenty, and their nuclei are greatly reduced in size on account of the 

 recent division (cf. d 6 ' 8 , Fig. 66, Plate X., with e? 7 - 15 , d 7 - 16 , Fig. 75, 

 Plate XL). The columnar form of the mother cells is retained by 

 their descendants. 



It has been already stated that the spindles in the endoderm cells 

 were at the recent division approximately horizontal in position. It is 

 evident, therefore, that before the accomplishment of division the attrac- 

 tion spheres must have shifted from the position which they were seen 

 to occupy in Plate X. Figs. 66-68, for otherwise the spindles would 

 have stood vertically, and a two-layei*ed arrangement of the cells would 

 have resulted, such as we shall see does occur in the case of the mesen- 

 chyme cells. No mechanical explanation of this change in the position 

 of the attraction spheres in the endoderm cells offers itself. The longest 

 axis of the cells appears to be continuously the vertical axis, vet the 

 spindles form in a direction transverse to this in every instance. Van 

 Beneden et Julin's ('86) Figures 1 c and 2 c also show spindles occupy- 

 ing the short axis of the endoderm cells in the case of Clavelina. 



Considering now the mesenchyme cells we see (Fig. 74) that D -8 , C 7 - 8 

 (Fig. 71) have divided in such a manner that a small superficial cell is 

 sepai'ated in each case from a many times larger deep-lying sister cell 

 (cf. Fig. 74, Z> 8 - 16 , Z>'- 15 , O 8,16 , C 8,15 ). A division similar in direction and 

 in the inequality of its products is foreshadowed for the next anterior pair 

 of mesenchyme cells (Fig. 75, e 7,12 , d 7 - 12 ), in which the spindles lie much 



