482 SCIENCE PROGRESS 



forms like the ants, ichneumon-flies, etc., the nucleolus is to be found within 

 the nucleus in much the same way as occurs in Patella or Saccocirrus ; we 

 find, too, that nucleolar buds pass into the egg-cytoplasm ; but the fate of 

 these buds is very different from what we have already seen. The most 

 wonderful part about the behaviour of the extruded nucleoli in ant-eggs is 

 that they form secondary nuclei ; that is to say, each nucleolar granule 

 expands and froths out to form a hollow sphere just like a true nucleus, and 

 with a nucleolus, nuclear membrane, and linin-network of its own. These 

 wonderful nucleolar nuclei — secondary or accessory nuclei, as they are known 

 to zoologists — often appear exactly similar to an ordinary nucleus, and we 

 have frequently deceived clever microscopists, who, on being shown such a 

 preparation, have asserted that the secondary nuclei were true nuclei. 



We have now obtained some conception of the peculiar diversity of 

 behaviour of the nucleoli of egg-cells — later we will return to the subject of 

 secondary nuclei, but at this stage it may be mentioned that the peculiar 

 secondary nuclei ultimately degenerate — leaving only the true nucleus 

 within the cell. 



Up to the present we have produced no evidence as to the probable relation- 

 ship between chromosomes and nucleoli. Until quite recently we really 

 had no evidence as to this question, which is one of great importance ; but 

 recently H. M. Carleton,^ working on the gut-cells of the frog and the cat, 

 has succeeded in breaking some interesting new ground. Rio Hortega,^ a 

 Spanish worker and a pupil of the famous Cajal, had previously shown that 

 the nucleoli of mammalian eggs contained small argentophile granules or 

 cores, which could be demonstrated by using a formalin silver nitrate 

 technique. 



Modifying such a technique, Carleton was able to show that this core, 

 during the division of the cell, behaved independently of the chromosomes ; 

 in cell division, as is well known, the chromosomes appear in the nucleus as 

 formed bodies which divide into two halves, each daughter cell receiving one- 

 half of the original chromosomes. Now, Carleton was able to stain the 

 chromosomes red in safranin or carmine, and the intra-nucleolar core or 

 nucleolinus, as he called it, a black colour in reduced silver ; the two sets of 

 bodies could not be confused, and the complete independence of both was 

 demonstrated without a doubt. We have ourselves gone through Carleton's 

 preparations. 



Let us now review the facts brought forward : the nucleolus of the egg- 

 cell may produce bodies of a proteid nature forming the egg yolk, it may 

 form nuclear -like bodies termed secondary nuclei, or it may produce fat in the 

 somatic cell (Schreiner). The independence of the nucleolus and the chro- 

 mosomes has been demonstrated clearly in a limited number of animals. 

 Obviously, we may say, the nucleolus is a separate and very important 

 nuclear element, whose functions in the metabolism of the cell are of signal 

 importance. 



Keeping these facts in view, and attempting to face the issues boldly, we 

 are led to look upon the nucleolus as a cell element, in the growth and life, 

 if not in the reproduction, of the animal — an importance equal to that of the 

 chromosomes themselves. The formation of secondary nuclei in ant and 

 other hymenopterous eggs is an occurrence which to us is fraught with 

 significance — the nucleolus, in other words, possesses the power of producing 

 nuclei independently of the chromosomes. 



The nucleolus to us represents the trophochromatin, the chromosomes 

 the idio- or gono-chromatin. Probably the nucleolus of the cell is the homo- 

 logue of the macronucleus of the protozoon like Paramoecium, the chromo- 

 somes, the homologue of the micronucleus. 



^ Quart. Jour. Micr. Science, 1920. 



* Trab, Lab. Invest. Biol. Madrid, xi, 191 3. 



