3o6 McMURRICH. [Vol. IV. 



perfectly normal, but it is certain that the refusion phenomena 

 are well marked. 



At the close of the two-celled stage the ovum is slightly- 

 oval, and shows no external trace of having consisted of two 

 distinct spherules. It then divides at once into four spherules, 

 the cleavage furrows arising at the exterior and passing towards 

 the centre, the segmentation belonging to that variety which 

 Metschnikoff distinguishes as centripetal {Z6). Two of the 

 spherules are usually smaller than the other two, and two lie 

 upon a different plane than the other two (Fig. 5). How this 

 is brought about I cannot say ; it may be due either to the 

 nuclei of the original spherules dividing obliquely to the plane 

 which originally separated them, or else to the nucleus of one 

 of the original spherules dividing in a plane at right angles 

 to that in which the other divides, as Ludwig has described in 

 Asterina ('82). 



Subsequent divisions lead to the formation of eight (Fig. 6), 

 sixteen, thirty-two, and sixty-four cells, although occasional irreg- 

 ularities were seen, such as stages in which there were seven 

 or eleven cells. The irregularity of the spherules evident in 

 the earliest stages becomes reduced as development progresses, 

 so that it becomes impossible to orient the embryos in later 

 stages, and so determine the relation of the axes of the ovum 

 and embryo. Inequalities are to be seen in the cells of young 

 blastulas (Fig. 7), but they are irregular, and do not serve to 

 mark out a pole of the embryo. The result of segmentation is 

 the formation of a hollow blastula, with walls composed of elon- 

 gated columnar cells richly packed with granules of food-yolk, 

 with a few clear vacuoles, and with their small nuclei situated 

 peripherally. This blastula is somewhat pyriform in shape (Fig. 

 8), and is ciliated, swimming about at the bottom of the vessel 

 in which it is contained, the narrower end being anterior, and 

 progression being accompanied by rotation about the long axis. 

 The cilia cover the entire body, and are equal in length except 

 at the pole which is anterior in swimming, where there is a tuft 

 of longer cilia, similar to what has been found in other Actinian 

 larvae. In sections of embryos in this stage the interior appeared 

 to be quite empty, but in others again (Fig. 11) it was filled 

 with what seemed to be a coagulable fluid, in which were scat- 

 tered granules of food-yolk evidently derived from the partial 



