MONOGRAPH OF THE EXISTING CRINOIDS. 401 



are more numerous and show a considerable increase in size, thougli very small 

 spherules are still abundant, and there is now a thin but distinct peripheral layer, 

 formed mainly of the larger spherules, which lies immediately on the inner side 

 of the vitelline membrane. From this time onward, as yolk formation progresses, 

 the peripheral sjDherules continue to increase both in size and in number, not 

 only in the body of the cell but also at the egg periphery. 



As yolk formation approaches completion the spherules scattered over the 

 cytoplasm decrease rapidly in number, though without showing any other change, 

 and by the time the process is completed have almost or entirely disappeared from 

 the body of the cell. At the same time, the peripheral layer increases considerably 

 in depth and density until, on the completion of yolk formation, it presents the 

 appearance Professor Chubb has already described in the preceding section and 

 which is maintained for as long as the egg remains in the pinnule. 



The behavior of the peripheral spherules just described clearly indicates that 

 they arise in the cytoplasm during yolk formation as a by-product of this process, 

 and gradually accumulate at the surface of the cell. They have thus nothing 

 to do with the discharged nucleolar spherules which, in the adult egg. they so fre- 

 quently resemble, and their very diiferent behavior with fixing reagents, especially 

 with sublimiite solution, serves to distinguish them from the often contemporane- 

 ously appearing chromatin spherules. The entire absence of any spatial relation 

 on the part of the peripheral spherules to the yolk nucleus, together with the 

 behavior already described for this structure, equally exclude the latter as a 

 possible source for these spherules. The frequency with which spherules, often 

 described as originating by the granular disintegration of the yolk nucleus, have 

 been made to take an active part in determining yolk formation necessitates con- 

 siderable emphasis being laid on the mode of origin and fate of the perijjheral 

 spherules in Anted on. 



Professor Chubb pointed out, when considering the significance of the nucleolus, 

 that one of the reasons for excluding the raw food material dissolved in the caryo- 

 lymph from the category of possible immediate sources for the nucleolar con- 

 stituents was the strict correlation the size of this structure invariably shows with 

 the phase of the egg's growth, a correlation very diflicult to understand, in the ab- 

 sence of any indication of a reaction between the nucleolus and the caryolymph, if 

 the growth of the nucleolus is due to the passive deposition on its surface of sub- 

 stance derived directly from a source outside the cell. AVhen, however, we come 

 to consider the behavior of the chromatin, we find that this structure, unlike the 

 nucleolus, does show a very obvious endeavor, which is indicated by its subdivision 

 and equal distribution, to increase the facilities for reaction with the surrounding 

 caryolymph. This behavior must indicate not merely that the chromatin depends, 

 for the material on which it works, on substances dissolved in the caryolymph, 

 for so does the nucleolus, but that these substances are not deposited, as such, on 

 the chromatin, but have instead to undergo their initial changes under the direct 

 influence of this cell structure while they are still dissolved in the caryolymph. In 

 other words, the chromatin obtains its food material by active incorporation, 

 whereas the nucleolus grows by passive deposition. The obvious source for the 



