6o8 



NATURE 



{Oct. 27, 1887 



cell in the body depend, and consequently the form and 

 function of the whole body are determined by the nuclear 

 matter or idioplasma of the first cell, the egg-cell— 

 parthenogenetic or fertilized. 



If this theoretical view is correct, then we must be 

 astonished that so much of this important nuclear sub- 

 stance is lost to the egg-cell— namely, one-half by the 

 parthenogenetic ovum, and half as much again by the 

 sexual one. What can be the cause that renders it 

 necessary for this to happen before the egg-cell is able to 

 develop into an embryo ? 



I will give a short account of my ideas upon the 

 subject. 



(i) The nuclear substance or idioplasma of the first 

 polar body must be detrimental to the further develop- 

 ment of the egg, for it is thrown out in all kinds of eggs, 

 parthenogenetic as well as sexual, and the embryonic 

 development never begins before the first polar cell has 

 been expelled. Now, if the nuclear substance truly con- 

 trols the cell and compels it to take a certain shape and 

 a certain histological structure, there must be such a 

 substance, such an idioplasma, also in the youngest egg- 

 cells. This idioplasma causes the t.zz to develop a yolk 

 possessing a certain colour and structure, it causes the 

 Qgg to form a shell of a certain thickness and structure ; 

 briefly, it compels the young egg-cell to attain a degree of 

 histological differentiation which it did not previously 

 possess. For the youngest egg-cells are essentially 

 similar in most animals, whilst mature egg-cells are very 

 different and can often be very well distinguished in 

 different species. The specific idioplasma of the grow- 

 ing egg-cell — I call it ovogenetic idioplasma — cannot be 

 the same as that contained in the nucleus of the mature 

 egg, and which controls the development of the embryo. 

 It cannot be that idioplasmi which determines the deve- 

 lopment of a certain e jg-cell into a duck and not into a 

 swan ; it cannot be that kind of idioplasma which I have 

 c&WeA. germ-idioplasma, or s\m^\y germ-plasma. 



Of course there must also be germ-plasma in t\it young 

 egg-cell; I beUeve that in the youngest germ-cells there is 

 no other idioplasma than germ- plasma, and that this germ- 

 plasma changes into ovogenetic plasma, only a very small 

 part of germ-plasma remaining unaltered. 



This remaining part grows with the growth of the egg, 

 and finally attains the same volume as the ovogenetic 

 idioplasma. Then the division of the nuclear substance 

 takes place, and the superfluous ovogenetic substance is 

 removed in the first polar globule, whereupon the egg-cell 

 contains only germ-plasma. 



This is my explanation of the removal of the yfr.y/ polar 

 cell. 



(2) In regard to the second it is clear that an egg that 

 contains only germ-plasma should be capable of under- 

 going embryonic development, unless the quantity of 

 germ-plasma should prove to be too small. But this is 

 not the case. Parthenogenetic eggs enter upon embry- 

 onic development immediately after the expulsion of the 

 first polar globule. Sexual eggs do not thus develop, 

 and we have to inquire into the reason for this. I be- 

 lieve it is because they throw out a second polar cell, 

 which takes away one-half of the germ-plasma left within 

 the egg-cell. After this the quantity of germ-plasma is 

 too small for entering upon embryonic development, and 

 therefore the egg-cell remains undeveloped, unless the 

 lost quantity of germ-plasma be replaced in the process 

 of fertilization. Embryonic development takes place 

 immediately after the union of the germ-plasma of a 

 spermatozoon with the remaining germ-plasma of the 

 ovum. Consequent upon this the quantity of germ-plasma 

 in a fertilized tgg again becomes equal to that which was 

 present after the separation of the first polar globule, 

 and also equal to that which enters upon embryonic 

 development in the parthenogenetic eg%. 



This is perfectly simple, but a great difficulty remains. 



Why is it necessary that the sexual egg should throw 

 out half of its germ-plasma ; why does it not retain the 

 whole quantity of this important substance ? 



You would perhaps answer. Because the quantities of 

 male and of female germ-plasma, that are united by 

 fecundation, must be equal. Indeed, the facts of heredity 

 lead to the opinion that these two kinds of germ-plasma 

 must be equal in quantity, and we have microscopical 

 observations recorded by Van Beneden, Carnoy, and 

 others, which further support this conclusion. But if the 

 quantity of germ-plasma must be equal in both, why 

 should the germ-plasma of the &gg increase so largely 

 as to attain twice the volume of the germ-plasma of a 

 spermatic cell ? Nature is not so wasteful as to throw 

 away so important a substance for nothing. There must 

 be an adequate cause why in sexual eggs the germ-plasma 

 must be halved before fecundation can take place. 



I believe I can point out the reason why this is 

 necessary, but before I do so 1 must beg you to first enter 

 with me upon a few theoretical considerations on the 

 subject of heredity. 



Heredity depends upon the germ-plasma, as I have said 

 before ; the minute molecular structure of the germ- 

 plasma causes the egg-cell to develop into a duck or a 

 swan, it also causes the egg to develop into a Negro or 

 into a European, into a Mr. Smith or into a Mr. Jones ; in 

 short, all qualities of the developed individual depend 

 upon the constitution of this germ-plasma. In my opinion 

 sexual propagation implies the union of two different 

 germ-plasmas to form the single nucleus of the egg-cell ; 

 and the two substances that are united in the process ot 

 fertilization I believe to be equal in size and quantity. 



Now let us suppose that we lived at a time when sexual 

 propagation had not yet existed, and that we were present 

 at its origin. We should then observe the union of two 

 different germ-plasmas, both of the same size and quantity, 

 but of a slightly different molecular constitution, one 

 coming from one parent and the other coming from 

 another. Both substances must be thoroughly homo- 

 geneous—that is to say, they must be composed of 

 particles that are equal in their chemical, molecular, and 

 morphological constitution. Let us illustrate this by a 

 diagram, in which we represent each germ-plasma as a 

 thread or a loop, which we know to be the microscopical 

 form of germ-plasma and of nuclear plasma in general. 

 For simplicity's sake we will represent only one loop for 

 the germ-plasma of each parent. We have then two 

 loops, the first representing the peculiarities of the germ- 

 plasma of one parent, and the second representing the 

 peculiarities of the other parent, and we will discriminate 

 between them by making >the first green and the second 

 red. 



These two individual kinds of germ-plasma unite and 

 form together the nucleus of the fertilized egg, which 

 develops into a new individual of the second generation. 

 This individual will form again germ-cells, and each ofj 

 these germ-cells will contain a germ-plasma, which is not! 

 homogeneous, as before, but composed of two halves,! 

 derived respectively from the two parents. In each suc- 

 ceeding generation the germ-plasma must attain to a[ 

 more complicated constitution, it must contain twice asj 

 many different kinds of germ -plasma as were contained j 

 in the germ-plasma of the preceding generation. If we| 

 follow this development of the germ- plasma for a few 

 generations, we shall find that union will take place '^ 

 sexual propagation between the germ-plasmas of two int. 

 viduals of the second generation, each containing two,, 

 different kinds of germ-plasma. In this way the indi-| 

 viduals of the third generation will be formed possessing 

 germ-cells which contain four different kinds of germ- 

 plasma. I have called these different kinds of germ- 

 plasma Ahttenplas7na, a word that can be rendered in 

 English by the term ancestral plasma. By sexual propa- 

 gation the individuals of the third generation would give 



