234 Chas. W. and G. T. Hargitt. 



together that some were pushed toward the outside, not reachmg the 

 cleavage cavity, and some toward the inside, not reaching the outer 

 surface. This gave to the blastula something of the appearance of 

 a double layer of cells. The cytoplasm was so filled with numerous 

 closely packed yolk granules that cell boundaries were indistinct; 

 the limits and number of cells must then often be estimated from the 

 number of nuclei. As nearly as could be estimated about 400 cells 

 are present in the blastula just before gastrulation. Figure 16 is 

 a section of an early blastula with rather large cells ; these by fur- 

 ther division become much more numerous and smaller. 



As pointed out by the senior author in a previous paper (1902 b, 

 p. 555) gastrulation in Cyanea may be easily observed in the liv- 

 ing egg to involve a very obvious invagination. This is just as 

 evident in Aurelia. One has only to remove some of the eggs to 

 a watch glass where the various phases may be followed from the 

 beginning to the closure of the blastopore. The entire process is 

 compassed in a comparatively brief time. 



Gastrulation 



Sections of Cyanea show that the cells of the blastula are longer 

 at one pole (figs. 15, 16) ; whether this is the animal or the vegetal 

 pole could not be determined on account of 'the absence of the polar 

 bodies at this stage. The first indication of gastrulation is the 

 flattening of a portion of one side of the blastula, usually the thin- 

 ner side as Hyde (1894) noted, followed by an invagination. Often 

 at the first the mouth of the invagination is broad and open, 

 but it soon narrows until the blastopore is almost closed (fig. 17). 

 The closure may be completed before the gastrulation is finished 

 (figs. 17, 18). When the invagination begins the coelenteron 

 may be broad, though shallow; as the blastopore closes the coelen- 

 teron becomes smaller and may be almost entirely absent (fig. -18). 

 Later this cavity again becomes more evident as is shown in the 

 figure of the cross section (fig. 19), the invaginated cells more 

 nearly filling the cleavage cavity. Still later these infolded cells, 

 which are the primitive entoderm cells, come to lie closely against 



