The Germinal Spot in Echinoderm Eggs. 



and appear as yolk-spherules scattered in a granulo-reticular matrix. The 

 nuclear reticulum has become coarser but is still pale-staining. Here and 

 there are scattered chromatin threads of beaded appearance. The nucleolns 

 is breaking up into many intensely chromatic globular bodies. 



Successively later stages show a continuation of the above process. 

 Figure 3 illustrates a stage where the nucleolar fragmentation and dispersal 

 has progressed a little farther. The nuclear reticulum is similar to its 

 appearance ^ in the last stage, except that there are fewer of the beaded 

 chromatin threads and, in this particular example, contraction of the nuclear 

 reticulum again produced a peripheral artifact. Examples could have been 

 selected where such contraction artifacts were lacking, but this particular 

 one was chosen to demonstrate by comparison with figures 2 and 4 that the 

 physical effects of the preserving fluid did not essentially modify the pro- 

 gressive changes in the nucleolar history. The cytoplasm now has the 

 appearance of mixed granular and alveolar type. The large yolk-spherules 

 have disappeared, probably by a transformation into fluid, a continuation 



FIG. i. Young ovum of Echlnaster crassisptna; nucleolus large, compact, homogeneous, 

 intensely chromatic and with sharp contour; nuclear reticulum pale and shrunken 

 away from wall at left; cytoplasm dark and coarsely granular. X 1500. 



FIG. 2. Nucleus at slightly later stage. Nucleolus breaking up into globular masses. 

 Nuclear reticulum pale, but through it are scattered chromatic beaded threads. A 

 portion of cytoplasm shown at left. It is still dark and granular, but many of the 

 granules (yolk) have greatly enlarged. X 1500. 



of which process for all the yolk-granules, large and small, culminates in the 

 beautiful alveolar cytoplasm of the ripe egg (fig. 7). Figure 4 gives a 

 stage in the dispersal of the nucleolar fragments. The products vary be- 

 tween wide limits, both in size and form, but most may be described either 

 as globes, dumb-bells, or tetrads. 



The culmination of the process of dispersal is illustrated in figure 5. 

 Figure 6 shows similar nucleolar fragments from a portion of a single 

 nucleus. Most of these have the form of typical tetrads. The unit of 

 structure here seems to be a globe and the individual mass a four-lobed 



