214 



NATURE 



\yuly 15, 1875 



formation with double sides ; these are the two original 

 germ-layers . The space enclosed by the inner germ- 

 layer is the intestinal cavity ; the whole formation we call 

 gastrula after Haeckel. As the causes of the formation of 

 the two germ-layers are the same for all animals con- 

 sisting of more than one cell (matazoa), according to 

 Gotte's view, the form of development of the gastrula is 

 therefore common to all, however indiscernible it may 

 often be in the outside appearance. The cause of this 

 is partly that the above-mentioned difference between 

 the upper and lower hemispheres of the ovum varies in 

 magnitude. If this difference is small, the result will be 

 that only a moderate part of the lower hemisphere will 

 be pressed inward, the .inner germ-layer remaining simple, 

 as for instance with the lower polypi, which on the whole 

 consist of two layers of cells. As the energy of the inward 

 pressure increases, a third germ-layer, the so-called middle 

 one, is split off the stronger inner one ; this third one, 

 from being a simple intermediary layer, may develop and 

 originate many and important organs. If in the dividing 

 ovum only the difference referred to in the vertical axis 

 exist, the gastrula is naturally formed equally in all direc- 

 tions between the two poles, so that if further transfor- 

 mations take place, these likewise occur equally in all 

 directions from the intestinal cavity and its principal axis, 

 and therefore in radiated planes or lines. Thus the 

 difference in the first axis of the ovum, if it acts by itself, 

 always leads to a radiated structure of body which we 

 find with Polypi, Medusae, Echinoidea, and their relatives. 

 Yet the higher developed representatives of these classes 

 already show here and there, and in unimportant points, 

 indications of a transition to a higher type. If we sup- 

 pose the two horizontal axes of the ovum to be unequal, 

 then the formation of the gastrula must naturally be un- 

 equal likewise. The inequality, which with many of the 

 Vermes already shows itself during the first divisions of 

 the ovum, causes the gastrula to extend in one direction 

 more than in any other, and thus to receive another prin- 

 cipal axis. If at the same time the two sides precede in 

 development the other parts, two symmetrical masses 

 are formed, situated opposite one another (germ-streaks), 

 and which approach each other more or less on the stomach 

 side, and there produce certain principal organs. To this 

 transverse divisions may be added,asin theArthropoda ; or 

 this may not occur, as in the MoUusca. Vertebrata finally 

 do not show the preponderance of the first formation on 

 two opposite symmetrical sides of the ovum, but only on 

 one, where the odd germ-streak is situated and indicates the 

 future back. In a manner similar to that of the typical 

 foundation of the embryo, Gotte tries to deduce all 

 other phenomena of development not from hypothetical 

 causes of inheritance, but directly from the laws of the 

 formation of the ovum ; as, for instance, the whole deve- 

 lopment of the different organs and tissues. Any essen- 

 tial change in a certain animal species must then be 

 deduced from a change in the laws of formation, which 

 are peculiar to the ovum, i.e. its first cause lies in the 

 ovum, and the live animal can never transfer newly- 

 gained changes of form directly to the law of formation 

 of its germs, nor thus cause its descendants to inherit 

 them. 



NEW DISCOVERY IN CONNECTION WITH 

 THE POTATO DISEASE 



npHERE has been hitherto one ''missing link" in our 

 ■■- knowledge of the life-history of the potato-blight, 

 Peronospora infestans. The non-sexual mode of repro- 

 duction by conidia or zoospores has long been known ; 

 but the sexual mode of reproduction has eluded observa- 

 tion. This link has now been supphed through the 

 researches of Mr. Worthington Smith, who described his 

 discovery in a paper read at the last meeting of the 

 Scientific Committee of the Royal Horticultural Society, 



and published at length in the Gardener's Chronicle for 

 July 10, He finds the female organs, the " resting- 

 spores " or unfertilised " oospores," and the male organs 

 or " antheridia," in the interior of the tissue of the tuber, 

 stem, and leaf, when in a very advanced stage of decay ; 

 and he has actually observed the contact between the two 

 organs in which the process of fecundation consists. In 

 some remarks made at the meeting of the British Asso- 

 ciation last year by one of our high authorities, it was 

 suggested that we have in the Peronospora an instance of 

 the phenomenon not infrequent among fungi, known as 

 " alternation of generations ;" and that the germination 

 of the true spores of the potato-blight must be looked 

 for on some other plant than the potato. Mr. Worthing- 

 ton Smith has, however, looked nearer home, and has 

 proved that the suggestion is not at all events verified in 

 all cases. It is matter of congratulation that, after the 

 lapse of a period of nearly thirty years since the publi- 

 cation of the first important memoir on the subject, this 

 discovery — important alike from a scientific and a prac- 

 tical point of view — has fallen to one of our own country- 

 men, notwithstanding the foreign aid invoked by the 

 Royal Agricultural Society in settling the still unsolved 

 problems connected with this perplexing pest. 



HISTORY OF THE PLAGIOGRAPH 



T SHOULD like to add a few words to my description 

 ■*■ of the instrument called the Plagiograph * (the g 

 to be pronounced soft, like 7, as in Genesis, Plagiarist, 

 Oxygen) in Nature, vol. xii. p. 168, for the purpose of 

 explaining the order of ideas in which it took its rise, 

 and also a very beautiful extension of another recent 

 kinematical invention to which it naturally leads the way, 

 and which, thus generalised, I propose to term the Quad- 

 ruplane. 



The true view of the theory of linkages \ is to consider 

 every link as carrying with it an indefinitely extended 

 plane, and to look upon the question as one of relative + 



* It may be questioned whether a new-born child can have a history. 

 Perhaps it might have been more correct to have used for my title, ' ' History 

 of the Birth of the Pl'\gicgraph," but this would have been long ; moreover, 

 the Plagiograph proves to be an unusually precocious child, having in its 

 very cradle given birth to a greater than itself, the Quadruplane, a full- 

 grown invention described in the sequel of the text. 



t It is quite conceivable that the whole universe may constitute one great 

 linkage, i.e. a system of points bound to maintain invariable distances, 

 certain of them from certain others, and that the law of gravitation and 

 similar physical rules for reading off natural phenomena may be the 

 consequences of this condition of things. If the Cosmic linkage is of the 

 kind I have called complete, then determinism is the law of Nature ; but, 

 if there be more than one degree of liberty in the system, there will be room 

 reserved for the play of free-will. We shoud thus revert to the Aristotelian 

 view under a somewhat wider aspect of circular (the most perfect because 

 the simplest form of motion) being the primary (however recondite) law of 

 cosmical dynamics. Speaking of cosmical laws brings to my mmd a reflec- 

 tion I have made upon the new chemical theory of atomicity. Suppose it 

 should turn out that the doctrine of Valence should be confirmed by expe- 

 rience, and that the consequent logico-mathematical theory of colligation 

 containing the necessary laws of consecution, or if cone pleases so to say 

 of cause and effect should plant its foot and introduce a firm basis of 

 predictive science into chemistry, how beautiful will be the analogy between 

 this and the physical law of inertia! which really merely affirms the fact 

 of each atom or point of matter carrying about with it a certain number, 

 denoting its communicative and inverse receptive faculty of motion ; for in 

 such case Valency, also affirming a numerical capacity for colligation, will 

 be the exact analogue in chemistry to Inertia in the theory of mass motion, 

 and might properly assume the name of chemical inertia. Social individuals 

 differ as egregiously as Isomers in their capacity for forming multifarious 

 attachments. 



% I believe it is to Mr. Samuel Roberts that we are indebted for the 

 idea of passing from mere copulated links to planes associated with the links, 

 and for the observation that the order of the corresponding Graphs is not 

 thereby augmented. The substitution of the more general idea of linkage 

 for link-work, and of isolating completely the conception of relative in lieu of 

 absolute motion, is due to the author of these lines. Take the case of a 

 Quadrupla«e in which the four joints in their natural order of sequence form 

 a contra-parallelogram. It is well known (and the fact has been applied to 

 machinery under the name of "the parallelogram of Reulleux") that the 

 relative motion of an opposite pair of planes maybe represented by causing 

 two curves to roll upon each other ; but I add that this may be done simul- 

 taneously for both pairs of planes, giving rise to a beautiful and previously 

 unthought of double motion of rolling (without slip) between two ellipses for 

 one pair and two hyperbolas for the other pair of planes. This is an imme- 

 diate deduction from the conception of purely relative motion. 



Note —In the description of the plagiograph, for pointed parallelo- 

 gram, p. 168, second c«lumn, line 14, lege jointed parallelogram. Also a 

 dotted line should be drawn in Fig. i connecting the points o and v'. 



