No. 3-] THE GASTRULATION OF AMPHIOXUS. 573 



blastopore was nearly closed, the figure shows the relative 

 growth of the dorsal and ventral walls. It will be noticed 

 that the ventral wall is longer than the dorsal, and this is 

 confirmed by other results. 



In one case that we have met with, an ectodermal pore was 

 present at the animal pole (PI. XXXIV, Fig. 18), yet the pres- 

 ence of this pore has not prevented the invagination of the 

 larger cells. If, as seems probable, the opening was present 

 during the gastrulation period, the embryo shows that the 

 process of gastrulation may take place even when a large pore 

 is present in the wall. That this is possible is also shown when 

 the vegetative pore is present, and yet invagination takes place. 

 Therefore, whatever mechanism is invoked to explain the proc- 

 ess of gastrulation, the process is of such a nature that it does 

 not demand a closed blastocoel space. 



At first the outline of the blastopore is oval (PI. XXXIV, 

 Fig. II). A large number of cells bound the opening, but, as 

 there is no sharp line of demarcation between the outer and 

 the inner invaginated cells, and since some of the cells at the 

 edge are partly within and partly without the rim of the blasto- 

 pore, the exact number and the shape of the boundary cells 

 cannot be accurately determined. In PI. XXXIV, Fig. 11, 

 about forty-two cells form the rim of the blastopore. At a 

 later stage the number is smaller. In PI. XXXIV, Fig. 12, 

 about thirty-four cells are around the margin. When the blas- 

 topore has further closed (PI. XXXIV, Fig. 13), twenty-eight 

 cells were counted. Finally, when the blastopore is reduced 

 in size, as shown in PI. XXXIV, Fig. 14, only ten cells were 

 present. 



How can we picture to ourselves the gradual reduction in 

 number of the cells as the blastopore becomes smaller .'' It 

 appears at times that, as the rim of the blastopore diminishes, 

 certain of the cells continue at the edge, but that others are 

 left behind in the general movement toward the center. In the 

 latest stage the boundary cells are elongated, and as a result a 

 larger number of cells surround the reduced blastopore than 

 would be the case if the cells all retained their earlier form. If 

 we judge by the shape alone of the blastopore, the closing takes 



