176 



NA TURE 



[December 22, 1892 



tion must be larger than those of either of the two substances 

 when separate, the solution consisting of compounds or hydrates 

 of the two. I showed, moreover, in the paper above re- 

 ferred to that the hydrate theory of solutions was quite capable 

 of accounting for and explaining the fact that the dissolved sub- 

 stance may f >r many purposes be regarded as being in a quasi- 

 gaseous condition in weak solutions, and that calculations based 

 on the idea of its being truly gaseous would yield very nearly 

 correct results. 



The hydraie theory will also, as I showed, give an explanation 

 of the fact that electrolytes will give abnormally high osmotic 

 pressures, and that the magnitude of these pressures can be cal- 

 culated from their electric conductivity : and the explanation 

 based on this theory also obviates many of the objections 

 to which the idea of dissociation into ions is open. More- 

 over, the only critical experiment which, as far as I 

 know, has ever been made to test the validity of 

 the dissociation hypothesis, gives an unequivocal answer 

 against it, and in favour of the hydrate theory. When, 

 for instance, sulphuric acid is dissolved in excess of water, it is 

 represented by the dissociationists as splitting up into its ions, 

 so that the solution will contain more acting units (ions and 

 molecules) than the acid and water together contained before 

 they were mixed : whereas, on the hydrate theory, combina- 

 tion will have occurred, and there will be fewer acting units 

 present. The nuuiber of acting units may be ascertained by ob- 

 serving the depression produced by the solution on some other 

 solvent, such as acetic acid (that is, by using the very method 

 which the dissociaiionists use to prove the supposed dissociation 

 of substances), and when this is done it is found that the sul- 

 phuric acid solution coxi\.z\n% fewer, instead of morcy units than 

 the acid and water separately. 



Even if the above were the only arguments to be urged, it is 

 evident that although the idea of the dissolved substance being 

 gaseous and often dissociated may be a good working hypothesis 

 for the directing of investigation, it can scarcely be accepted as 

 a true theory of the nature of solutions. 



Spencer Pickering. 



On a Supposed Law of Metazoan Development. 



It is difficult not to feel disappointed that Dr. Beard has given 

 only "a preliminary sketch byway of clearing the ground" 

 (Nature, vol. xlvii. p. 79), in place of "producing the full 

 argument" for a law in the existence of which he has by 

 '* observation and reflection " been led to believe. For it is not 

 easy to gather from his sketch how he is able to apply a universal 

 law to so varied a series of events and phenomena such as he 

 mentions, and at the same time to point out " the analogy which 

 obtains between the suggested mode of Metazoan development, 

 and the accepted fact of an alternation of generations in the life 

 histories of all plants above the lowest Thallophytes." For in 

 the higher plants the alternation of generations referred to occurs 

 with constancy as regards period of life history, and varies only 

 slightly within the limits of the same group. 



Dr. Beard alludes, I presume, to one form of alternation of 

 generations — that of sexual with asexual generation only, or 

 Metagenesis. This he asserts constitutes a general law in the 

 development of Metazoa. 



In a sense this may be true enough. If, for instance, we re- 

 gard the division of each cell as a new asexual generation, then 

 Metagenesis is a very constant phenomenon amongst Metazoa. 

 In this case the life history of a Metazoon consists of a sequence 

 of thousands of asexually produced generations alternating with 

 one sexually produced generation, which gives apparently a 

 stimulus for another run of asexual generations in which poly- 

 morphism and division of labour are exhibited in extraordinary 

 complexity and beautiful harmony. 



But this is not at all what Dr. Beard means. The series of 

 instances which Dr. Beard gives, or system of " nursing " as 

 Steenstrup termed it, is at most a series of disconnected 

 phenomena of frequent occurrence, and not a law. 



Because most Metazoa possess eyes, it is not therefore a law 

 of Metazoan development that eyes should be developed. 

 Diversity in form, number, and time of appearance of eyes, 

 is sufficient to show that the law cannot exist ; so also is it in 

 the cases of nursing to which Dr. Beard alludes, and on which 

 he bases his argument. 



It seems to me that no "law " of alternation of generations 

 in Metazoa can be " enunciated " unless there is evidence forth- 



NO. l?o8. VOL. 47I 



coming of its constant action at corresponding periods in the 

 life histories of all animals of different groups, and in a closely 

 similar manner in individuals of one and the same group. Also^ 

 a law of such a nature, if it is to be found to act universally 

 amongst Metazoa, must surely have come into action at a very- 

 early period in the evolution of Metazoa. 



Metagenesis is of constant if not universal occurrence in the 

 cycle of life of Protozoa. A long series of generations pro- 

 duced asexually is followed by a generation produced sexually, 

 that is, a generation produced by the conjugation of two indi- 

 viduals ; this is followed again by another long series of 

 asexually produced generations, and so on. If this is so constant 

 among unicellular organisms of the present day, it is not very 

 unreasonable to suppose it was common among the protozoan 

 ancestors of the Metazoa and of the Plants. If we are to find 

 any form of Metagenesis as a universal phenomenon in the 

 Metazoa, it must be to the most protozoon-like stages of 

 development of the Metazoon that we should look. ■ 



There is but one strict meaning to the phrase sexual 

 generation, and that is a fusion of two cells. If Metagenesis 

 means anything it means the alternation of a generation resulting 

 from the fusion of two cells, with one or more generations re- 

 sulting from the division of cells. 



This we can perhaps find in the protozoon-like stages of 

 Metazoan development, and in a way analogous to the alternation 

 of generations among plants. 



Spermatozoon and ovum fuse and form the fertilized ovum 

 which is the true sexually produced generation. This produces 

 by division a vast number of cells, and if we regard these as a 

 number of generations then Metagenesis is obvious enough. 

 But it is no more metazoic — if I may u^e such a word — to call 

 the whole animal resulting from the segmentation of the 

 fertilized ovum, the sexually produced generation. 



This generation buds off the immature ovum. This is really 

 the "Primitive ovum" of the embryo. I see no reason why 

 this may not be regarded as a distinct asexually produced 

 generation — like the formation of the spore of the plant. 



The immature ovum divides into two cells — first Polar body, 

 and more mature ovum. The more mature ovum divides into 

 two cells, namely, second Polar body and mature ovum. It 

 does not materially affect the argument whether we should re- 

 gard these two processes as two separate consecutive asexually 

 produced generations, or as one asexually produced multi- 

 cellular generation. If we take the latter view, then the 

 maturation of the ovum is more analogous to the prothallus 

 stage of the life history of plants. 



In either case the result is the formation of the mature ovum, 

 comparable to the oosphere of plants. 



The mature ovum fuses with the mature spermatozoon, and 

 the sexually produced generation recurs, and the cycle of 

 development is completed. 



I cannot help thinking that if Dr. Beard wishes to discover a 

 law of Alternation of Generations applicable to the whole of the 

 Metazoa, he will find a more favourable hunting ground amongst 

 those stages of development at which the several groups of 

 Metazoa approximate, than amongst those stages where they 

 are farthest apart ; and also Dr. Beard will find the analogy 

 between the supposed Metazoan law and the accepted law of 

 the vegetable kingdom closer than he could ever hope to find 

 it if he continues his present line of search. 



If the above theory of the cycle of Metazoan life can be con- 

 sidered tenable, we see that both in the Hi.;her Plants and in the 

 Metazoa there jire constantly alternating " sporophyte " and 

 " gamophyte " generations, and further, we can find evidence, 

 as we should expect to do, of the origin of such a universal 

 phenomenon in the single celled or protozoan life, where the 

 continuance of the species may be secured in both these ways, 

 namely, by the formation of asexually produced spores, and as- 

 a consequence of cell fusion, i.e. conjugation. 



RiC. ASSHETON. 



Oxygen for Limelight. 

 The employment of oxygen for limelight and other purposes 

 has increased enormously since the commercial introduction of 

 the Brin method, by which the gas is separated from atmospheric 

 air by a now well-known chemical process. The gas so obtained 

 is practically pure, analysis showing that as now supplied by the 

 Brin companies it contains on an average 95 per cent, of 

 oxygen, the remaining five per cent, consisting of inert nitrogen. 



