THE PYCNOGONIBS. 15 



Internal Changes. 



Sections of the early stages of segmentation give little infor- 

 mation in addition to what we see from surface views. They 

 show that each nucleus is contained in a separate segment, and 

 that many of these segments run to the center of the egg. Those 

 cells derived from the first micromere cannot reach to the center 

 of the sphere. 



Figrl-, Plate I, is a section of an egg at the stage when there 

 are thirty-two micromeres. The upper part of the figure is 

 probably in the region of the micromeres. In the center of the 

 egg appears a clear cavity without yolk, but this is not constant 

 for all eggs. In some it is certainly absent, but I am unable to 

 say whether or not it is caused by hardening reagents. What 

 seems to be a similar cavity is found in the eggs of some spiders. 

 The whole egg is divided up into the segments in the form of 

 pyramids. Only some of these in this figure can be traced to the 

 center, but other sections show each pyramid of the macromeres 

 to have its apex at or near the center of the egg. The nuclei 

 are situated around the periphery of the section, and the proto- 

 plasm, which invariably accompanies each nucleus, sends out 

 fine pseudopodial filaments into the surrounding yolk. 



Fig.-ffcis in the same stage as the last, and undoubtedly passes 

 through the micromeres at the upper pole. These micromeres 

 and their nuclei are seen to be smaller than the cells elsewhere, 

 and the pyramids are seen to fall short of the center of the egg. 

 There is no central cavity present in this egg. 



Sections of eggs somewhat older than the last show that the 

 nuclei of the upper pole have rapidly increased, and that they have 

 migrated to the surface of the yolk, which loses its pyramidal 

 structure over the upper surface. The protoplasm surrounding 

 each nucleus comes into contact with that of surrounding nuclei, 

 though how closely such fusion is formed I cannot say. Such a 

 condition is shown in Fig. 3, where the protoplasm forms a cap 

 over the upper surface of the yolk. The nuclei in this section 

 are abnormally large, which is probably due to the hardening 

 reagents, but the section seems normal in other respects. The 

 lower area of the same egg shows three or four scattered nuclei 

 near the surface of the yolk. At this stage of development there 

 is a single layer of cells at the surface of the yolk. Between this 



