CTENOPHORA 139 



in endoplasm (en) filling the inside. The latter is foamy, 

 ►wing to the existence of a large number of spherical 

 vacuoles, between which only a scant reticulation and mesh- 

 work of protoplasm (formative yolk) is left. It is very 

 probable that these spherical vacuoles are all filled with a 

 homogeneous, slightly refractive mass, which has sub- 

 stantially the characters of food-yolk. Therefore in what 

 follows we shall occasionally designate the entire endoplasm 

 merely as food-yolk mass. The gerrainative vesicle is found 

 in the superficial cortex of ectoplasm (Fig. 64). 



Even though the cleavage in the Ctenophora, as we shall 

 see, exhibits its peculiarities, yet we can on the whole desig- 

 nate it as total, unequal cleavage, which leads to the forma- 

 tion of an epibolic or circuracrescence gastrula. However, 

 this latter type of gastrulation is not preserved in its 

 purity, for eventually an invagination process participates 

 in the sinking of the entoderm into the embryo. 



The first furrows that make their appearance are to be 

 designated as meridional, inasmuch as they cut through 

 from the animal to the vegetative pole in the direction of 

 the subsequent longitudinal axis. By means of the first of 

 these furrows the egg is divided into two equal parts (Fig. 

 65 J) ; by means of the second furrow, likewise extending 

 in a meridional direction and perpendicular to the first, 

 ther'e are formed four cleavage spheres, arranged crosswise 

 (Fig. 65 B, F) ; these are oriented in such a manner as re- 

 gards the embryo resulting from them that each cleavage 

 sphere corresponds to one quadrant of the embryo (FoL, No. 

 7). The third act of cleavage leads to the appearance of 

 additional meridional furrows, which, as the dotted lines in 

 Fig. 65 F indicate, make an angle of 45 degrees with those 

 already present. If this cleavage were to take place regu- 

 larly in the way indicated, eight large cleavage spheres of 

 equal size lying in one plane would result. On the con- 

 trary, the eight-cell stage presents a variation from this 

 regularity which recurs in all Ctenophora, and is import- 

 ant for the subsequent formation of the embryo. For the 

 furrows that now make their appearance are shifted in 

 such a manner that, as is indicated by the dotted lines in 



