No. 3-] THE GASTRULATION OF AMPHIOXUS. 585 



eight body cavities are given off ; the cells that form the noto- 

 chord, and the mesodermal pouches are filled with yolk gran- 

 ules, and the ectoderm is also. This reappearance of the yolk 

 in the later stages occurs after the blastopore has been closed, 

 and at a time when all the regions of the body are easily dis- 

 tinguishable. What we have stated in regard to the earlier 

 stages, throughout which the yolk has been gradually disap- 

 pearing from certain cells and remaining constant in others, is 

 not effected by these later changes. We feel, therefore, no 

 less certain of our results, based, as they are, largely on the 

 distribution of the yolk and the histology of the cells during 

 the early stages. 



Irregularities in the Endoderm. 



In looking for the vegetative pore as a means of orienting 

 the gastrula stages, we soon found that, at other places, 

 depressions occurred in the endoderm resembling the point 

 at which the vegetative pore closes (PI. XXXIII, Fig. 9). 

 We had, therefore, to abandon this landmark as a means of 

 orientation. Often we have found that irregularities in the 

 endoderm are due to the rounding up of cells during the final 

 stages of division. In other cases, as in PI. XXXIV, Fig. 20, 

 at X, the nuclei are in a resting condition, but this may be 

 interpreted to mean that after the division the nuclei have 

 become spherical, but the cells have not yet lost their rounded 

 contour. This view is supported by the fact that two such 

 cells are generally found together. In the ectoderm, too, we 

 have often seen two dividing cells showing the same tendency 

 to become spherical during division. However, cells in all 

 phases of division may be found that have not rounded up as 

 described, yet even in these cases the inner free ends of the 

 cells are rounded, thus breaking the even contour of the 

 endoderm. 



Oblique Sectiojis through Blastopore. 



It has been pointed out that the early gastrula is somewhat 

 asymmetrical (PI. XXXIII, Fig. 2), the invagination being 



