PRESIDENTIAL ADDRESS. 41 1 



with cytoplasm. A great deal of confirmatory evidence can be brought in 

 favour of the view that the cytoplasm of the egg is at first homogeneous but is 

 modified as growth proceeds by the agency of material emitted by the nucleus. 

 Thus if the unfertilised egg of the Nemertine worm C'erebratuhis be broken 

 into fragments and spermatozoa added, the fragment which contains the 

 nucleus alone will develop into a larva. If, however, we wait until the nuclear 

 membrane has dissolved and the contents of the nucleus have diffused into the 

 cytoplasm, then, when the egg is broken into fragments and the fragments 

 fertilised with spermatozoa, each will develop into a larva. It is obvious that 

 the whole quality of the cytoplasm has been changed by what has been dis- 

 charged into it from the nucleus. And the same thing can be observed in the 

 egg of Ascaris. We have just learnt that this egg when ripe has its cytoplasm 

 sharply differentiated into zones, one of which contains the peculiar substance 

 which determines diminution of the chromatin. But if the unripe eggs of 

 Ascaris be subjected to centrifugal force, they can lose large portions of their 

 cytoplasm and yet the diminished remnants containing the nuclei, if fertilised, 

 will produce perfectly normal embryos of reduced size, showing that when the 

 egg is unripe the cytoplasm is perfectly homogeneous. We are all aware that 

 Weismann in his famous theory of the Germ-Plasm anticipated many of these 

 conclusions. He also regarded the peculiar cytoplasmic qualities of the various 

 cells of the body as caused by the emission of peculiar materials from the 

 nuclei, but there is one fundamental difference between Weismann's theory and 

 the view which we have been led to take as a result of all the experiments 

 which have been described. Weismann supposes that the division of the 

 nucleus, though it results in the formation of two apparently similar daughter- 

 nuclei, is in reality in many cases a differential division and separates two 

 different kinds of chromatin : and that the differences in the cytoplasms of 

 various cells which become obvious as development processes are due to differ- 

 ences in the constitution of the nuclei which they contain. He supposes that 

 the nuclei of certain cells from the beginning of development retain the con- 

 stitution of the nucleus of the egg and that some of the descendants of these 

 cells do the same and eventually give rise to the germ-cells, and he termed the 

 supposititious pre-determined lineage of cells leading from the fertilised egg to 

 the germ-cell a germ-track : these germ-tracks are then imagined to stretch 

 in a continuous chain from generation to generation, transmitting their 

 characters unaltered, whereas the other cells which constitute the body are a 

 sort of by-product of these. Now, we have seen that it has been experimentally 

 demonstrated that the nuclei in the blastula of the sea-urchin and in the earlier 

 segmentation stages of the frog's egg are alike and can be interchanged with 

 one another with impunity, and yet at the very period of the development at 

 which this obtains most definite and distinct cytoplasmic differentiation can 

 occur — at any rate in the frog's egg ; therefore we are led to agree with 

 Hertwig that all the nuclei of an embryo are potentially alike, and that 

 in the case of many animals definite pre-determined ' germ-tracks ' do not 

 exist. Quite recently evidence strongly confirmatory of this view has been 

 brought forward by Gatenby. This observer finds that in the common frog 

 every season a new generation of egg-cells is formed by the modification of 

 ordinary peritoneal cells. Previous observers had traced the first origin of the 

 germ-cells back to a very early stage in the development of the tadpole and had 

 maintained the existence of definite germ-tracks in this animal. But Gatenby, 

 whilst admitting the truth of their observations, points out that these primitive 

 germs would not suffice for the supply of eggs even for the first spawning 

 season, and that the much more numerous eggs that are spawned in subse- 

 quent seasons are derived by the gradual modification of typical peritoneal 

 cells, and that the first indication of this modification consists in the appear- 

 ance of a blush of chromatin surrounding the nucleus — a blush which we may 

 surely interpret as an emission of organ-forming materials into the cytoplasm. 

 We have so far discussed the appearance of organ-forming substances as if 

 they were elaborated and discharged from the nucleus solely during the period 

 of the ripening of the egg. This appears to be the case in such highly 

 specialised eggs as those of Ctenophores, Mollusca, and Nematoda, but we have 

 to consider the case of eggs like those of the sea-urchin and star-fish, which are 



