THE PYCNOGONIDS. 11 



be due in part to the segments of the egg struggling to give 

 synchronous beats at each rhythm, but being hampered by the 

 presence of the yolk. If the first plane of segmentation of Pallene 

 corresponds with the first plane of division of Phoxichilidium and 

 Tanystylum, and & priori this seems most probable, is it not con- 

 ceivable that the acquisition of yolk to one-half of the egg might 

 cause great changes in the synchronism of segmentation of the 

 two parts ? and if so, it is easy to imagine that we have here an 

 egg in a period of variation, and that one or another of the above 

 changes may become fixed for the species. 



Later stages of segmentation than those described are difficult 

 to follow or to distinguish between macromere and micromere 

 of the upper pole, but in general the cells around the upper pole 

 are smaller and more numerous than at the lower. Fig. J, 

 Plate III, shows the ventral (or macromere) hemisphere of an 

 egg at a later stage than any of the preceding. The outer ends 

 of the cells, those seen in the figure, are polygonal, and the nuclei 

 lie very near to the surface. The whole cell is pyramidal in 

 shape, with its apex at the center of the egg and its polygonal 

 base at the periphery of the egg^ with the nuclei in the base 

 of the pyramids. Each nucleus is still accompanied by a 

 surrounding mass of protoplasm, and it is without doubt this 

 protoplasm which is seen from surface views. At this time the 

 cells of the lower pole rarely divide, and remain for a long time 

 almost constant in size and number, but the cells at the upper 

 pole undergo rapid changes ; and our attention must now be 

 turned almost entirely to that region. 



The next change which we can see in surface views is at the 

 upper pole, where a whitish opaque area is forming, and which may 

 be profitably compared to the similar formation in spiders' eggs and 

 called there the Primitive Cumulus. This is due to an accumu- 

 lation of cells at this point, where an invagination is about to 

 take place to form the stomodseum. Soon there are other opaque 

 areas formed at the surface, which are seen to occupy definite 

 positions and are the thickenings for the brain, ventral ganglia and 

 appendages. These are shown in Figs. I and II, Plate IV. In 

 Fig. I w T e have a surface view of the head region of a young 

 embryo. In the upper part of the figure are first the two oval 

 thickenings to form the brain or supra-cesophageal ganglion. 



