156 



NATURE 



[Dec. 17, I ; 



these, and it would appear as if the two nuclei could not 

 differ, but must be exactly identical. Strasburger, there- 

 fore, considering this identity a fundamental fact, concludes 

 that the difference between the two must arise subsequently 

 to their separation as the effect of unlike nutrition. It is 

 urged by Weismann, in antagonism to this view, which 

 would be fatal to his theory, that all that is really proved 

 by the fact is that at every division of a nucleus an equal 

 mass of maternal and paternal nuclear substance passes 

 to form each daughter nucleus, but that it is by no means 

 shown that the quality of the parent nuclear plasma must 

 be identical on both sides. On the other hand, from the 

 effect of the daughter nuclei on their respective cell- 

 bodies, which are most commonly different both in size 

 and texture, it seems proved that they are usual Iv 

 different in quality. As well-marked examples may be 

 cited the polar vesicles. In the case of some Mollusca 

 the eg^ gives off, by the indirect method of nuclear divi- 

 sion, two polar vesicles, one after another, and each of 

 these divides into two. The four polar vesicles perisli, 

 whilst the nucleus of the ovum remaining in the yolk 

 combines with the sperm nucleus, and, making use of its 

 own cell body, becomes the embryo. The reason for tlie 

 difference here must be that the quality of the nucleus 

 of the polar body is different from that of the ovicell. 



In accordance with Nageli's views, then, the molecular 

 structure of the germ-plasma must be so much the more 

 complicated the more complex the organism is which 

 is to be developed out of it : and further, it can be 

 stated that the nuclear substance must become suc- 

 cessively less and less complicated as ontogeny proceeds, 

 in proportion as the foundations which yet have to be 

 evolved out of any cell and of which the nuclear plasma 

 is the molecular expression, becoine gradually less in 

 number. The general nucleo-plasma becoming thus 

 gradually more and more simple in molecular structure, 

 soon loses its capability of reproducing the entire 

 organism ; it cannot by any process be metamorphosed 

 back again into the immensely complicated germ-plasma. 

 Only the nucleo-plasma of the original segmentation 

 nucleus is germ-plasma — that is, possesses the structure 

 by the regulating action of which on the process of growth 

 the entire organism can be evolved. In many cases, 

 from the moment of the first division of the ovum into 

 two blastomeres the one blastomere loses the power of 

 reproducing the whole organism out of itself alone, since 

 one resulting blastomere represents the future epiblast, 

 the other the hypoblast. Somatic nucleo-plasma cannot 

 become converted into germ-plasma. 



Phylogenetically the germ cells did not originate at the 

 termination of ontogeny, but at its commencement, as is 

 well shown by the conditions existing amongst certain 

 lower chlorophyll containing organisms such as Pandorina 

 and Volvox. The phyletic origin of the first germ cells 

 must evidently be sought amongst the earliest polycellular 

 organisms differentiated by division of labour. In the 

 genus Pandorina of the Volvocinejc no such division of 

 labour has as yet arisen : each spherical colony is com- 

 posed of exactly similar flagellate cells (each with an eye- 

 spot, chlorophyll contents and pulsatile vacuole) embedded 

 in a common colourless jelly {Jioiiwplastuic). These colonies 

 reproduce themselves alternately by asexual and sexual 

 process ; in the latter case the copulating individuals are 

 not yet distinguishable from one another as male and 

 female ; in either case every cell of the colony remains as 

 yet a complete unicellular organism capable of separate 

 reproduction. In V'olvox, another genus of the same 

 family, a hctcroplastid condition has been attained and 

 the separation into somatic and generative cells has been 

 effected. The spherical colony consists of two sorts of 

 cells— numerous small flagellate cells and much fewer 

 large germ cells devoid of flagella. The latter alone 

 can effect the production of a new Volvox sphere and can 

 do this in two ways, either asexually (parthenogenetically) 



or after impregnation by small actively-moving sperma- 

 tozoa formed out of certain of their own number. Now, 

 as Kirchner has shown, the germ cells become separated 

 off from the somatic cells early in the segmentation of the 

 volvox ovum before the escape of the young heteroplastid 

 from the egg-coverings, which is exactly as should be 

 according to Weismann's views. 



Here is proof that there is no intervention of somatic 

 cells in the course of growth between the parent germ- 

 cell and the daughter germ-cells, but that the latter arise 

 directly from the parent germ-cell, and thus thecontinuity 

 of the germ plasma is established as a fact for the com- 

 mencement of the phyletic development series. In later 

 times, with increasing complexity of the organism, the 

 time of the separation of the germ-cells became gradu- 

 ally more and more postponed in most cases, and at the 

 present period often occurs quite late at the end of the 

 entire ontogeny. If in the egg of Diptera the first two 

 nuclei which separate themselves by division from the 

 segmentation nucleus of the egg form the reproductive 

 cells, this is a proof that they receive the entire germ- 

 plasma of the segmentation nucleus unchanged. 



There are, however, scarcely any theoretical grounds 

 against the supposition that unmodified germ-plasma 

 might be mingled with the nuclear substance of the 

 somatic cells ; on the other hand, it would appear 

 a priori very conceivable that all somatic cells might 

 contain some unmodified germ-plasma. The fact that a 

 complete Begonia plant with fruit and fertile seed can be 

 grown from a Begonia leaf, whilst in the case of many 

 other plants no such result can be obtained, seems to 

 show that in certain plants the cells, or perhaps only 

 certain cells, of the leaf contain germ-plasma, whilst 

 in others unchanged germ-plasma is not present in the 

 leaves at all, or in very minute quantities only. In the 

 case of the mosses, where almost every cell of the roots, 

 leaves, and axial shoots can become a complete plant, 

 probably all, or nearly all, the component cells must con- 

 tain an adequate supply of germ-plasma. 



The Meaning of the Polar J'esie/es. — The egg-cell must 

 contain two kinds of nucleo-plasma or idioplasma, 

 namely, germ-plasma and histogenetic plasma. During 

 its growth it has to accumulate yolkand toforin surround- 

 ing membranes, in soine cases to form a micropyle, and 

 otherwise adapt its cell-body to the production of the 

 future embryo. Therefore, besides the germ-plasm it 

 carries, it requires another kind of specific nucleo-plasma 

 just like every other histologically differentiated cell. 

 This histogenetic plasma cannot be the same as that 

 which subsequently governs the development of the em- 

 bryo, and which arises from the infinitely complex germ- 

 plasma. As soon as the egg is ripe for fertilisation it is 

 necessary that the histogenetic plasma should be got rid 

 of, in order to leave the germ-plasma free to act, and the 

 extrusion of the polar bodies is the removal of this oi'o- 

 genoi/s nucleo-plasma. This is an entirely new theory 

 as to the significance of the polar bodies, and directly 

 opposed to all those which would see the extrusion of a 

 male element in the act. 



In the case of the male sperm cell, also, two kinds of 

 nucleo-plasma are present — germ-plasma and spermo- 

 genous nucleo-plasma. As soon as the spermatozoon 

 is ripe, the spermogenous nucleo-plasma is cast off 

 as the equivalent of the polar body. Strasburger has 

 lately described a large number of instances amongst 

 plants of different groups, in which processes resembling 

 the extrusion of polar bodies accompany the ripening of 

 the generative elements of both sexes. And it is probable 

 that similar conditions will in time be discovered to exist 

 in other plants. 



On tlie Nature of Parthenogenesis. — The fact of the 

 formation of the polar bodies, considered in the light of 

 the theory of the sexuality of the germ cells, has been 

 freely made use of to e.xplain the occurrence of partheno- 



