332 



NA TURE 



[August 2. 1900 



plain that the physical basis of inheritance is in the fertilised 

 ovum. As regards property, there is an obvious distinction 

 between the inheritance and the person who inherits, but there 

 is no such distinction in biology. The fertilised egg-cell is the 

 inheritance, and is at the same time the potential inheritor. 



An organic inheritance means so much, even when we use the 

 magic word potentiality, that we may consider for a moment the 

 difficulty which rises in the minds of many when they remember 

 that the egg-cell is often microscopic, and the sperm-cell often only 

 l/ioo,oooth of the ovum's size. Can there be room, so to speak, 

 in these minute elements for the complexity of organisation .sup- 

 posed to be requisite? The difficulty will be increased if the 

 current opinion be accepted that only the nuclei within the 

 germ-cells are the true bearers of the hereditary qualities. 



In reference to this difficulty, it may be recalled that the stu- 

 dents of physics tell us that the image of a Great Eastern filled 

 with framework as intricate as that of the daintiest watches does 

 not exaggerate the possibilities of molecular complexity in a 

 spermatozoon, whose actual size may be less than the smallest 

 dot on the watch's face. Secondly, as we learn from embryology 

 that one step conditions the next and that one structure grows 

 out of another, we are not forced to stock the microscopic germ- 

 cells with more than initiatives. Thirdly, we must remember 

 that the development implies an interaction between the grow- 

 ing organism and a complex environment without which the 

 inheritance would remain unexpressed, and that the full-grown 

 organism includes much that was not inherited at all, but has 

 been acquired as the result of nurture or external influence. 



The central problem of heredity is to form some conception 

 of what we have called the relation of genetic continuity 

 between successive generations ; the central problem of inherit- 

 ance is to measure the resemblances and differences in the 

 hereditary characters of successive generations, and to arrive, if 

 possible, at some formula which will sum up the facts. There- 

 fore, while it is interesting to ask how an organisation supposed 

 to be very complex may be imagined to find physical basis in 

 a microscopic germ-cell, the same sort of question may be raised 

 in regard to a ganglion-cell. It is not distinctively a problem 

 of heredity. Similarly, while it is interesting to inquire into the 

 orderly and correlated succession of events by which the fer- 

 tilised egg-cell gives rise to an embryo, this is the unsolved 

 problem of physiological embryology. 



In the preformationist theories, which asserted the pre- 

 existence ot the organism and all its parts, in miniature, within 

 the germ— there was a kernel ot truth well concealed within a 

 thick husk of error. For we may still say that the future or- 

 ganism is implicit in the germ, and that the germ contains not 

 only the rudiment of the adult organism, but the potentiality of 

 successive generations as well. But what baffled the earlier in- 

 vestigators was the question how the germ-cell comes to have 

 this ready-made organisation, this marvellous potentiality. 



An attempt to solve this difficulty of accounting for the com- 

 plex organisation presumed to exist in the germ-cell is expressed 

 in a theory which occurred at intervals in the long period be- 

 tween Democritus and Darwin, the theory of pangenesis. On 

 this theory, the cells of the body are supposed to give off 

 characteristic and representative gemmules ; these are supposed 

 to find their way to the reproductive elements, which thus come 

 to contain concentrated samples of the different components of 

 the body, and are, therefore, able to develop into an offspring 

 like the parent. The theory involves many hypotheses, and is 

 avowedly unverifiable in direct sense-experience, but it is more to 

 the point to notice that there is another theory of heredity which 

 is, on the whole, simpler, which seems, on the whole, to fit the 

 facts better, especially the fact that our experience does not 

 warrant the conclusion that the modifications or acquired 

 characters of the body of the parent affect in any specific and 

 representative way the inheritance of the offspring. 



As is well known, the view which most biologists now take of 

 the uniqueness of the germ-cells is expressed in the phrase 

 " germinal continuity." There is a sense, as Mr. Galton says, 

 in which the child is as old as the parent, for when the parent's 

 body is developing from the fertilised 'Ovum, a residue of un- 

 altered germinal material is kept apart to form the future repro- 

 ductive cells, one of which may become the starting-point of a 

 child. In many cases, from worms to fishes, the beginning of 

 the lineage of germ-cells is demonstrable in very early stages 

 before the differentiation of the body-cells has more than begun. 

 In the development of the threadworm of the horse, according 

 to Boveri, the very first cleavage divides the fertilised ovum into 



NO. 1605, VOL. 62] 



two cells, one of which is the ancestor of all the body-cells, and 

 the other the ancestor of all the germ-cells. In other cases, 

 particularly among plants, t\ie segregation of germ-cells is not 

 demonstrable until a relatively late stage. Weismann, generalis- 

 ing from cases where it seems to be visibly demonstrable, main- 

 tains that in all cases the germinal material which starts an 

 offspring owes its virtue to being materially continuous with the 

 germinal material from which the parent or parents arose. But 

 it is not on a continuous lineage of recognisable germ-^r^/A that 

 Weismann insists, for this is often unrecognisable, but on the 

 continuity of the getm-plasin — that is, of a specific substance of 

 definite chemical and molecular structure which is the bearer ot 

 the hereditary qualities. In development, a part of the germ- 

 plasm, "contained in the parent egg-cell is not used up in the 

 construction ot the body of the offspring, but is reserved un- 

 changed for the formation of the germ-cells of the following 

 generation." Thus the parent is rather the trustee of the germ- 

 plasm than the producer of the child. In anew sense, the child 

 is a chip of the old block. The conception of a germ -plasm is 

 hypothetical, just as the conception of a specific living stuff or 

 protoplasm is hypothetical. In the complex microcosm of the 

 cell, we cannot point to any one stuff and say, " this is proto- 

 plasm " ; it may well be that vital activity depends upon several 

 complex stuffs which, like the members of a carefully constituted 

 firm, are characteristically powerful only in their inter-relations. 

 In the same way, we cannot demonstrate the germ-plasm, even 

 if we may assume that it has its physical basis in the stainable 

 nuclear bodies or chromosomes. The theory has to be judged, 

 like all conceptual formulae, by its adequacy in fitting facts. 



II. Dual Nature of Inheritance. 



Apart from exceptional cases, the inheritance of a multi- 

 cellular animal or plant is dual, part of it comes from the mother 

 and part of it from the father. 



Prof. E. B. Wilson states the general opinion of experts some- 

 what as follows : — As the ovum is much the larger, it is believed 

 to furnish the initial capital — including it may be a legacy of 

 food-yolk — ^for the early development of the embryo. From both 

 parents alike comes the inherited organisation which has its seat 

 (according to many) in the readily stainable (chromatin) rods of 

 the nuclei. From the father comes a little body (the centro- 

 some) which organises the machinery of division by which the 

 egg splits up, and distributes the dual inheritance equally between 

 the daughter-cells. 



Recent researcties confirm a prophecy which Huxley made in 

 1878 : " It is conceivable, and indeed probable, that every part 

 of the adult contains molecules derived both from the male and 

 from the female parent ; and that, regarded as a mass of molecules, 

 the entire organism may be compared to a web of which the 

 warp is derived from the female and the woof from the male." 

 "What has since been gained," Prof. Wilson says, "is the 

 knowledge that this web is to be sought in the chromatic sub- 

 stance of the nuclei, and that the centrosome is the weaver at 

 the loom." 



In regard to these conclusions, three notes are necessary, (a) 

 Although inheritance is dual, it is in quite as real a sense multi- 

 ple, from ancestors through parents, (b) If Loeb is able to in- 

 duce artificial parthenogenesis in sea-urchins' eggs exposed for a 

 couple of hours to sea-water to which some magnesium chloride 

 has been added ; if Delage is able to fertilise and to rear normal 

 larvae from non-nucleated ovum-fragments of sea-urchin, worm 

 and mollusc, we should be chary of committing ourselves defi- 

 nitely to the conclusion that the nuclei are the exclusive bearers 

 of the hereditary qualities, or that both must be present in all 

 cases. Furthermore, the fact that an ovum without any sperm- 

 nucleus, or an ovum-fragment without any but a sperm-nucleus, 

 can develop into a normal larva points to the conclusion, pro- 

 bable also on other grounds, that each germ-cell, whether ovum 

 or spermatozoon, bears a complete equipment of hereditary 

 qualities. [c) It must be carefully observed that our second 

 fact does not imply that the dual nature of inheritance must 

 be patent in the full-grown offspring, for hereditary resemb- 

 lance is often strangely unilateral, the characters of one parent 

 being "prepotent," as we say, over those of another. 



III. Different Degrees of Hereditary Resemblance. 



One step of progress during the Darwinian era has been the 

 recognition of inheritance as a fact of life which requires no 

 further proof. 



Yet this aspect of the study of heredity is by no means worked 



