THE PYONOGONIDS. 17 



Keeping before us the process of delanrination in Phoxichi- 

 lidium, etc., I think we may regard these inner nuclei of Pallene 

 to have come from the outer cells by delamination, and even 

 that we may push the comparison a step farther and consider 

 that each cell of the outer layer to have given rise at one time 

 in its history to an inner cell, and then that the outer cells con- 

 tinued to divide tangentially to form the blastoderm. The 

 reasons for such a belief are these : — In cross-section the number 

 of inner nuclei are slightly in excess of the peripheral nuclei of 

 the lower pole (see Figs. 4 and 5). As the outer cells of the 

 upper pole were at the beginning more numerous than the peri- 

 pheral cells of the lower pole, we ought to get, if the hypothesis 

 be true, exactly what we do find. Further, at the periphery of 

 the blastoderm, where the inner cells of the lower pole are added 

 on, we can always see such a method of multiplication taking- 

 place. The differences between this process and that in Phoxi- 

 chilidium are these : that multipolar delamination does not take 

 place simultaneously in all the cells at once, but in Pallene 

 slowly progresses as the cap of cells makes its way to the lower 

 pole of the egg, incorporating into itself as it goes the outer cells, 

 which cells, as they are added on first, give off an inner cell. 



After this stage we pass to older embryos, where these two 

 layers — ectoblast and entoblast — begin to differentiate into 

 organs. The first to appear is the stomodseum, which results 

 from an invagination of ectoblast. This is shown by Fig. 17, 

 Plate II. Here it is seen the ectoblast at one point has pushed 

 inwards, and around the periphery of the invagination appear 

 several cells with branching and anastomosing pseudopodia. 

 These I believe to be the first appearance of the mesoblast. A 

 few of the cells drawn in the figure belong without doubt to the 

 entoblast, and at this stage it is difficult to separate the two. The 

 section passes longitudinally through the embryo, and just ventral 

 to the stomodseum there is a thickening of the ectoblast to form 

 the first ventral ganglion. Under the ectoblast are found much 

 branched entoblast cells, which are still comparatively few in 

 number. Whether the circumstomodasal mesoblast comes from 

 ectoblast or entoblast I am unable to say. Dorsal to the epiblast 

 the epidermis continues conspicuously for a short distance and 

 then becomes exceedingly thin. 



