410 TRANSACTIONS OF SECTION D. 



to be cited as the most convincing proof that the differentiation of the germ is 

 the result of the differentiation of the nuclei. For when it divides into two cells 

 the nucleus of the upper cell undergoes the remarkable change known as 

 DiMiNTjTiON OF THE CHROMATIN. There are at most four chromosomes in the 

 fertilised egg : in the upper cell just after division a large portion of these is 

 cast forth into the cytoplasm and absorbed, whilst the remainder breaks up into 

 a large number of minute chromosomes. The upper cell gives rise only to 

 ectoderm, whereas the lower gives rise to all the internal organs. Now, if an 

 egg happens to be fertilised by two spermatozoa, a curious monster results, 

 which may have any one of three forms in the four-cell stage. It may consist 

 of two upper and two lower cells, and in this case it will develop into a complete 

 twin embryo • it may consist of one upper and three lower cells — in this case it 

 will develop into a monster with three sets of internal organs ; or, finally, it may 

 consist of three upper cells and one lower cell, in which case it will develop 

 into a fairly normal embryo with an unusually voluminous amount of ectoderm. 

 Now, Boveri, by an exhaustive analysis, shows that the assumption that the 

 cause of the diminution of chromatin lies in the nucleus leads to conclusions 

 •which are totally at variance with the facts : that it must lie in some peculiar 

 substance collected in one region of the cytoplasm ; and that the different 

 results obtained by double fertilisation are due to the accident that, of the four 

 nuclei resulting from the first cleavage, one, two, or three may lie in the region 

 containing this substance. But the most convincing proof is furnished by an 

 ingenious experiment which we have been able to repeat in the laboratory of the 

 Imperial College. If the fertilised eggs of Ascaris be fixed to a slide and put 

 into a centrifugalising machine and a high speed of rotation be maintained 

 for a considerable time, and the eggs divide into two whilst undergoing this 

 rapid rotation, then it will chance that the planes of division of some of them 

 from their position on the slide will be exactly radial. When this occurs both 

 cells are exactly alike — neither nucleus undergoes diminution, and each cell gives 

 rise to a set of internal organs ; but the least obliquity of the plane of this 

 division to the axis of rotation results in the formation of two cells, one of 

 which exhibits diminution of chromatin in the normal manner, and gives rise 

 to the ectoderm, whereas the other nucleus remains unaffected and the cell con- 

 taining it gives rise to the internal organs. We may assume that the peculiar 

 substance which causes diminution is driven to the outer part of the egg by 

 the centrifugal force, but it is impossible to avoid the conclusion that, in an egg 

 the plane of whose first division lies slightly oblique with regard to the axis of 

 rotation, both the first cells must receive some of the substance, and yet only 

 one nucleus undergoes diminution. Therefore the fact that one cell receives 

 more of the substance than its fellow must determine the diminution of the 

 chromatin and its subsequent development. 



Having studied the general properties of these marvellous substances so far 

 as the evidence at our disposal will permit, we must try to find out something 

 of their origin. In the case of the egg of the A'scidian Cynthia the origin 

 of one of them at least is obvious. For, as we have seen, the ectoderm owes its 

 origin to the nuclear sap. But a little reflection will render it clear that in the 

 last resort all these organ-forming substances must arise from the chromatin. 

 For the father's contribution to the fertilised egg is merely a small mass of 

 chromatin — the spermatozoon head — and yet organs are inherited from the 

 father just as well as from the mother. Now, Schaxel has shown that when 

 the unripe egg is examined it is possible by appropriate methods of staining 

 to detect streams of chromatin gi-anules both inside and outside the nuclear 

 membrane, forming in many cases accumulations against the nuclear membrane 

 and pointing in the clearest manner to the conclusion that chromatin material is 

 being poured into the cytoplasm and is modifying its character. This is 

 especially obvious in the unripe egg of Cynthia. Even the nuclear sap must be 

 regarded as a by-product of the chromatin : for Gates has shown that when, as 

 hajjpens in the ripening of the pollen-cells of Oenothera, a piece of chromatin 

 becomes detached from the nucleus of one cell and discharged into the cyto- 

 plasm of its neighbour, this piece acts like a miniature nucleus and surrounds 

 itself with a nuclear membrane inside -which is nuclear sap. It is thus seen 

 that nuclear membrane and sap are both produced by the reaction of chromatin 



