226 CORA J. BECKWITH 



tinia a membrane is also present. Sections of an unfertilized egg 

 show a yolk-free area at the surface of the egg in which mitochon- 

 dria are scattered irregularly (fig. 41) . No distinct membrane can 

 been seen at this time. Sections of fertilized eggs show the mito- 

 chondria arranged in a distinct layer directly at the surface of 

 the egg, a more or less free space being left between them and the 

 yolk-spheres. Outside the layer of mitochondria a very thin, 

 transparent layer or membrane appears, which as a rule clings 

 closely to the surface of the egg (fig. 42) . The method of formation 

 of the membrane was not determined. 



5. Nucleolus 



As stated earlier, a nucleolus is already present in the egg after 

 the last oogonial division, when the chromatin is in the bouquet- 

 stage (figs. 1, 2, 3). Its origin was not determined and it stains 

 intensely in basic dyes. As the chromatin breaks up into a net, 

 it is still pressed against the nuclear wall and may already show 

 vacuoles (fig. 4). When the radial arrangement of the net ^ is 

 lost, the nucleolus is no longer flattened against the nuclear wall, 

 but may be well toward the center of the nucleus (figs. 7, 18). In 

 general, however, it retains its excentric position (figs. 19, 21, 22). 

 During the growth-period the nucleolus increases in size and 

 becomes vacuolated, the skeleton retaining- its intense staining 

 capacity for basic dyes while the vacuoles stain with acid dyes. 

 One large vacuole may occupy the center, leaving a rim of basic 

 staining material (fig. 29 b), or many fine vacuoles may appear, 

 making the nucleus more or less spongy (figs. 27, 28, 29 a). Both 

 conditions are typical for Hydroids. The increase in size continues 

 throughout the greater part of the growth-period, a point in which 

 Hydractinia differs from Pennaria, as described by Hargitt, since 

 in the latter form growth stops and nucleolar disintegration begins 

 as soon as the spireme is completely broken up. About the time 

 of maturation — usually before the breaking down of the nuclear 

 membrane — the nucleolus dwindles and disappears in the sub- 

 stance of the nucleus by a process of dissolution and not by frag- 

 mentation as described by Hargitt for Tubularia. It may have 



