ESSAYS 481 



In the ordinary body- or somatic-cell the function of the nucleolus is 

 almost completely unknown. Schreiner ^ believes that he has been able to 

 show that the nucleolus of fat- cells buds off small fragments which, passing 

 into the cytoplasm, give rise to fatty granules. In such a case it is evident 

 that the nucleolus shares largely in the fat-metabolism of the cell. We are 

 inclined to accept Schreiner's account, especially in view of what we know 

 happens in the case of fatty granules in certain eggs. 



During the life of the cell, in some cases the nucleolus seems to act as 

 a peculiar centre in the prophases of mitosis. Some observers have been led 

 to look upon the nucleolus as a store of chromatin which is given up at each 

 mitosis to the forming chromosomes, and in tissue-cultures of live cells which 

 we have examined it seems true that the nucleolus does take some part as a 

 centre for the aggregation of chromosomes — but not necessarily as a store- 

 house for chromatin. 



The latter view does not commend itself to us. When, however, we come 

 to the question of the egg cell, we find that much more is known of the func- 

 tions and behaviour of the nucleolus. In the very young egg cell the nucleolus 

 is normal, and of the same size as that of the typical connective-tissue cell. 

 During the growth of the egg, however, most remarkable changes may 

 overtake the original nucleolus, its contribution to the formed elements of the 

 cytoplasm may be very considerable, and the changes undergone by the 

 fragments of nucleolus within the cytoplasm may be of a remarkable 

 description. 



The first case which we may consider is that of the egg of Patella. The 

 latter is the well-known limpet found on rocks about the level of the sea- 

 shore when the tide is half-way out. The case of Patella has recently been 

 worked out by R. Ludford, who has kindly allowed us to see his preparations. 

 In the young egg of Patella, the nucleolus is oxyphil— a plasmosome, or, in 

 other words, it stains red in eosin, and not blue or basophil in haematoxylin. 

 As the cell grows the plasmosome becomes slightly fragmented, and the 

 secondary nucleoli so found are shot out of the nucleus into the ground-cyto- 

 plasm of the cell. So far as has been ascertained these extruded elements 

 soon degenerate, but a peculiar change gradually occurs in the original 

 nucleolus : part of the latter loses its affinity for the red dye, and begins to 

 stain blue in the haematoxylin ; one then has formed a double body or 

 amphinucleolus, part red or oxyphil, and part blue or basophil. The red 

 part goes on budding oS smaller pieces, which pass into the egg-cytoplasm, 

 while the blue half does not seem to take much part in this process. 



So far as can be stated in such a diificult matter, the portions of the 

 nucleolus extruded into the egg-cytoplasm take no part in the formation of 

 yolk or fat or other formed granules or deutoplasm of the Patella egg. The 

 fragments appear to disintegrate gradually. In such a case as this, we find 

 it very difficult to bring forward an explanation as to the purpose of these 

 nucleolar extrusions. 



When, however, we examine the oogenesis or egg-formation of a marine 

 worm like Saccocirrus, we find a very different story. In Saccocirrus we have 

 a nucleolus inside the young egg-nucleus just as with Patella, but during the 

 growth of the egg, the original nucleolus buds off fragments which — passing 

 into the cytoplasm — give rise by fission to numberless granules of a dense 

 proteid nature ; these granules form the main bulk of the reserve materials of 

 the Saccocirrus egg, and constitute the yolk. In such a case there is no doubt 

 as to the function of the nucleolus — it is here an organ for the production of 

 reserve materials, which are later to be used up in the development of the 

 embryo. 



In the egg-formation of animals there is still another peculiar type of 

 nucleolar behaviour which we may notice. In the insects, and especially in 



^ 4nat. Anzeig. (191 5). 



