1 62 BIOLOGICAL LECTURES. 



Stand in a genetic connection with that early stage of the yolk 

 substance, when the latter cannot as yet be differentiated by 

 staining methods. In other words, it would seem that the yolk 

 is at first present in the cytoplasm in the form of a diffused, 

 unstainable fluid ; that a portion of it, that remaining in the 

 cell body, later becomes segregated as, or chemically changed 

 into, yolk globules ; and that another portion of it is taken into 

 the nucleus and, after passing the nuclear membrane, changed 

 into nucleolar substance. 



The objection might be raised by those who assume the 

 nuclear origin of yolk, that the nucleoli are yolk globules formed 

 within the nucleus and destined to be expelled into the cell 

 body. But two answers may be given to such a criticism. 

 First, we have seen that the first discernible yolk globules 

 within the cytoplasm lie near the periphery of the cell, conse- 

 quently at some distance from the nucleus ; and, second, the 

 broader reason, that the ovarial ovum is essentially charac- 

 terized by its rapid growth, and that this growth is maintained 

 necessarily by accretion of nutriment from without ; the cyto- 

 plasm derives its nourishment from the body cavity or blood 

 vessels, and the nucleus from the cytoplasm. This being the 

 case, and the fact recalled that increase in nucleolar substance 

 appears to always accompany rapid growth of the nucleus, leads 

 to the conclusion that nucleolar substance stands in a pretty 

 close relation to nutritive metabolism ; while the fact that 

 nucleoli first appear at the periphery of the nucleus speaks for 

 their extranuclear origin. 



There can be no doubt that the physiological processes of 

 nucleus and cytoplasm stand in a most intimate relation, and 

 possibly even that the nucleus directs and controls the cyto- 

 plasmic processes. Accordingly, there are good reasons for 

 concluding that the formation of yolk globules may be directed 

 by the nucleus. But the point for which I am contending is 

 that the yolk substance, which is undoubtedly nutritive and 

 hence in the first place extracellular in origin, is not produced 

 but only chemically changed and segregated by the action of 

 the nucleus. And since, in the germinal vesicles of Nemerteans 

 and nudibranch mollusks at least, the nucleolar substance shows 



