August 1 6, 1877] 



NA TURE 



305 



line of illustrious vegetable physiologists have brought to light 

 the details of the process by which fertilisation is effected, and 

 have shown in fact how the minute tube developed from the 

 inner membrane of the poUen-pranule, as soon as it falls upon 

 the stigmatic tissue of the seed-bearing plant, insinuates itself 

 by a rapid process of development between the cells of the style, 

 and reaches at last the ovule, in the interior of which is the 

 embryo-sac ; how, having passed into the micropyle, or orifice of 

 the ovule, it makes its way to the embryo-sac ; how a minute 

 portion of the ftrtilising substance of the fovilla transudes from 

 the pollen tube into the cavity of the embryo-sac, in which by 

 this lime a certain portion of the protoplasm has become differ- 

 entiated into the germinal vesicle, thereby stimulating it to 

 further growth and development, the earliest phenomena of 

 which manifest themselves by the formation of an investing 

 cell- wall, and by the occurrence of cell-division, which results in 

 the formation of the embryo or plantule of the seed. 



Thus it appears that the essential part of the process of pro- 

 duction in phanerogamic plants is the formation in the parent 

 plant of cells of two different kinds, which by themselves have 

 little or no independent power of further growth, but which, by 

 their union, give rise to a product in which the power of 

 development is raised to the highest degree. 



By further researches it is now known that the same law 

 prevails in all the remaining members of the vegetable kingdom, 

 with the exception only of the very simplest forms.' 



In viewing the reproductive process in the series of crypto- 

 gamic plants, two facts at once strike us as remarkable in the 

 modifications which are observed to accompany the formation of 

 a productive germ, viz. : First, that the difference between the 

 two productive elements becomes, as it were, more prominent, 

 or more highly specialised, in the cryptogamic than in the 

 phanerogamic plants ; and second, that in the simpler and 

 lower forms this difference gradually disappears till it is lost in 

 complete uniformity of the productive elements. 



Thus in the whole trdie of the ferns and vascular crypto- 

 gams, in the liigher algce and fungi, in the characeii; and in 

 the mosses, the differentiation of the productive elements is 

 carried to a very high degree ; for while that belonging to the 

 embryo or germ presents the structure of a simple cell which 

 remains at rest, or in a comparatively passive state, and, 

 absorbing into itself the substance of the other, becomes the 

 seat of subsequent development ; the other, corresponding to 

 the pollen of the staminiferous phanerogam, is usually separated 

 from the place of its formation, and having undergone a peculiar 

 modification of structure by which it acquires active moving 

 cilia, it changes place, and is directed towards the germinal 

 structure, and coming in contact with its elementary cell, is 

 more or less absorbed, or lost in the fertilising process. Tlie 

 protoplasm of the germinal cell, thus acted on and fertilised, 

 then proceeds to undergo the changes of development by which 

 the foundation is laid for the new plant. 



In the algie and fungi, however, there are gradations of the 

 differentiation of the two reproductive cells, which are of the 

 greatest in'erest in leading to a comprehension of the general 

 nature of the formative process. For in the lower and simpler 

 forms of these plants, such as the Desmidi^, Mesocarpeas, and 

 other Conjugate, we find that there is no distinction in structure 

 or form to be perceived between the two cells which unite or 

 undergo conjugation ; and a complete fusion or intermixture of 

 the two masses of protoplasm results in the production of a 

 single, usually spherical, mass holding the place of an embryo. 

 And that there is an absence of specialisation between the two 

 uniting cells is clearly shown iir both Dcsniuiiuiii SMi. Miwcarpus, 

 by the fact that the embryo or zygospore is formed in the mass 

 resulting from the union of the protruded portions of the two 

 cells ; and in more ordmary cases, as in Spirogyra, where the 

 embryo is formed in one of the two cells, it seems to be 

 indifferent in which of them it is formed. 



From this, which may be regarded as the most elementary 

 type of new production by the union of the two cells, the transi- 

 tion is not a great one to the development of a progeny without 

 any such union. We might conjecture, then, that the capacity 

 for separate or individual existence extends in the lowest 

 organisms to the whole, or to each structural element of their 

 organisation, while as we rise in the scale of vegetable life (and 

 the same view might apply to the animal kingdom) this capacity 



■ It will be ohser^eJ ll|;.t I leave entirely out of view the whole subject 

 of the mullpllcation of I^lants by budding or simple division. 



is more and more divided between the two productive elements, 

 or, at least, is only called into full action by their combination. 



The germinal element consists of a simple primordial cell, 

 varying in different kinds of plants, but in all of them probably 

 containing the essential substance protoplasm, and the most 

 immediate result or effect of fertilis.ation is the multiplication by 

 repeated fissiparous division of the previously-existing cells. 

 Tlie new individual resulting from this cellular growth usually 

 remains within the parent body, without, however, direct union 

 or continuity of tissue, till the embryo has attained some 

 advancement, as in the well-known case of the seeds of a phane- 

 rogam ; but there are many varieties in the mode of its disposal 

 among the lower plants. 



A remarkable exception to the more direct relation of the 

 process of fertilisation to the formation of the new individual or 

 embryo occurs in some plants, simulating in some respects that 

 kind of variation in animal reproduction which has been named 

 alternate generation. A well-known instance of this belongs to 

 the vascular cryptogams. The prothallium of the ferns, for 

 example, results from the development of so-called spores or 

 unicellular buds, which are familiar as being formed in small 

 capsules on the lower leaf-surface ; and in this prothallium, when 

 it has reached a certain stage of vegetation, there are formed the 

 archegonia, containing the oospheres, or germ cells, which are 

 fertilised by the moving ciliated particles developed in the cells 

 of the antheridia, leading to the production of a new spore- 

 bearing plant. 



Recent researches have also called attention to the remarkable 

 arrangements in phanerogamic plants for the prevention of 

 fertilisation of the pistils by pollen from the same flower, or even 

 from the same plant. In the latter case this is effected by the 

 separation of stamens and pistils in different flowers on the same, 

 or on different plants. In the former case, where both organs 

 occur in the same flower, the adapta' ions, whether of a mechanical 

 or of a physiological character, by which self-fertilisation is 

 prevented, as ascertained by numerous recent investigations 

 (among which those of Darwin are most conspicuous), are of the 

 most varied and often the most complicated kind. 



Let us now turn to the consideration of the development of 

 animals, and let me say in the outset that it will be necessary for 

 me to confine my remarks chiefly to the higher or vertebrated 

 animals, and to certain parts only of the history of their develop- 

 ment ; more particularly the structure and formation of the ovum 

 or egg, its earlier developmental changes, and the relation of 

 these to the formation of the new animal. 



I cannot enter upon the consideration of this topic without 

 adverting to the very recent acquisition of some of the most im- 

 portant facts upon which this branch of knowledge is founded ; 

 and I feel it to be peculiarly appropriate, in the year of his 

 death, to refer to a biologist whose labours contributed more 

 powerfully than those of any other person to give to animal 

 embryology the character of a systematic branch of science, and 

 to whom we owe some most important original discoveries, — I 

 mean Karl Ernst von Eaer of Konigsberg, St. Petersburg, and 

 Dorpat. 



Of observers who, previous to von Baer, were mainly instru- 

 mental in preparing the way for the creation of a more exact 

 modern science of embryology only two can be mentioned, viz., 

 Caspar Frederick Wolff" of St. Petersburg, well known as the 

 author of a work entitled " Theoria Generationis," published in 

 ' 759) t'y which the epig^nesis^ or actual formation of the organs 

 in a new being, was first demonstrated, and Christian Pander, 

 who by his researches made at Wurzburg explained, in a work 

 published in 1817, the principal changes by which the embryo 

 arises and is formed. 



Von Baer was born in the Russian province of Esthonia on 

 February 29, 1 792. After h.aving been fifteen years professor in 

 the Prussian University of Konigsberg, he was called to St. 

 Petersburg, and having some years later been appointed to a 

 newly-established professorship of Comparative Anatomy and 

 Physiology, he remained in that city for nearly thirty years as the 

 most zealous and able promoter of scientific education and 

 research, stimulating and guiding all around him by his un- 

 exampled activity, comprehensive and original views, sound 

 judgment, and cordial co-operation. In 1S6S, at the age of 

 seventy-six, he retired to Dorpat, from the University of which 

 he had received his degree in 1814, and continued still to occupy 

 himself with working and writing in his favourite subjects, as 

 well as interesting himself in everything that was related to 

 educational and scientific progress, to very near the time of his 



