3o6 



NATURE 



\Augtist i6, 1877 



death, which occurred on November 2S, 1876, in his eighty-fifth 

 year. 



Although von Baer's researches, according to the light in 

 which we may now view them, contributed in no small degree to 

 the introduction of the newer views of the morphological relations 

 of organic structure which have culminated in the Theory of 

 Descent, yet he was unwilling to adopt the v'ews of Darwin ; 

 and one of his latest writings, comjileted in the last year of his 

 life, was in vigorous opposition to that doctrine. 



It would have been most interesting and instructive to trace 

 the history of the progress of discovery in embryology from the 

 period of von Baer down to the present time, but such a history 

 would not be suitable to the purpose of this address ; and I can 

 only venture here, in addition to Ratbke, the colleague of Baer 

 in Kunigsbevg, to select two names out of the long list of dis- 

 tinguished workers in this field during the last forty years, viz. : 

 Thomas Bischoff, of Giessen and Munich, to whom we owe the 

 greatest progress in the knowledge of the development of mam- 

 mals, by his several memoirs, appearing from 1842 to 1S54 ; and 

 Robert Remak, of Berlin, whose researches on the development 

 of birds and batrachia, appearing from 1S50 to 1855, gave 

 greatly-increased exactness and extension to the general study of 

 development. 



The germinal element, from which, when fertilised, the new 

 animal is derived, is contained within the animal ovum or egg — 

 a compact and definite mass of organic matter, in which, not- 

 withstanding great apparent variations, there is maintained 

 throughout all the members of the animal kingdom, excepting 

 the protozoa, which are destitute of true ova, a greater imiformity 

 in some respects than belongs to the germinal product of 

 plants. 



Usually more or less spherical in form, the animal ovum 

 presents the essential characters of a "complete cell," in the 

 signification given by Schwann to that term. The germinal 

 substance is inclosed by an external vesicular membrane or cell- 

 wall. Within this covering the cdl-subsianct^ generally named 

 yolk or vitellus, from the analogy of the fowl's egg, corrsists, to 

 a greater or less extent, of a mass of protoplasm, and imbedded 

 in this mass, in a determirjate situation, there is found a smaller 

 internal vesicular body, the gcniiiual-vt'sicli: or nucleus, with its 

 more or less constant or variable viaciila or nucleolus. 



Now the first thing which strikes us as renraikable connected 

 with the ovum is the very great variation in size as compared 

 with the entire animal, while in all of them the same simple or 

 elementary structure is maintained. The ovum of mammals is, 

 for example, a comparatively small body, of which the average 

 diameter is about the xiul'i' o*^ ^^i inch, and which consequently 

 scarcely weighs more than a very minute fraction of a grain, 

 which may be calculated perhaps only at the , .; JsTith part. And 

 further, in two animals differing so widely in size as the elephant 

 and the mouse, the weights of which may beheld to stand towards 

 each other in the proportion of 150,000 to I, there is scarcely 

 any difference in the size of the mature ovum. 



On the other hand, if we compare this small ovum of the 

 mammal with the yolk of the egg in the common fowl, the part 

 to which it most nearly corresponds, it may be estimated that 

 the latter body would contain above three millions of the smaller 

 ova of a mammal. 



The attribute of size, however, in natural objects ceases to 

 excite feelings of wonder or surprise as our knowledge of them 

 increases, whether that be by familiar observation or by more 

 scientific research. We need not, at all events, on account of 

 the apparent mir;uteness of the ovum of the mammrfer, or of any 

 ether animal, have any doubts as to the presence of a sufficient 

 amount of germinal substance for explaining in the most material- 

 istic fashiorr the transmission of the organic and other properties 

 and resemblances betw een the parent and offspring. For we are 

 led to believe, by those who have recently given their attention 

 to the size of molecules composing both living and dead matter, 

 that irr such a body as this minute ovum of the mammal, there 

 may be as many as five thousand billions of molecules, and even 

 if we restrict ourselves to the smaller gerrrrinal vesicle, and, 

 indeed, to the smallest germinal particle which might be made 

 visible by the highest microscopic enlargement, there are still 

 sufficient molecules for all the requirements of the most exacting 

 material biologist.^ 



^ According tu a calculation made by Mr. Scrby, the number of mole- 

 cules in the germinal vesicle of the mammalian ovum is such that if one 

 molecule were to be lost in every second of time, the whole would not be 

 exhausted in seventeen years. See Address to the Micrcscopic Society in 

 Jettrtt, 0/ Miavsccp. Sairia; vol. xv. p. 225, and Naturh, vol. xiii. 



This great disparity of size is, however, connected with an 

 important difference in the disposition of the yolk-substance, 

 according to which ova may be distinguished as of two kinds — 

 the large- and the small-yolked ova, between which there are 

 also many intermeiliate gradations. The larger yolked ova 

 belong to the whole tribe of birds, scaly reptiles, osseous and 

 cartilaginous fishes, and the ccphalopods among the inverte- 

 brates ; and are distinguished by the strictly germinal part or 

 protoplasm being collected into a small disc, known familiarly 

 as the cicatricula of the fowl's egg, and to be seen as a whitish 

 spot on that side of the yolk which naturally floats uppermost ; 

 while the rest of the yolk, of a deeper yellow colour, contains a 

 large quantity of vitelline granules or globules of a different 

 cheinical nature from the protoplasm. 



The phenomena of embryonic development are, in the first 

 instance at least, 'confined to the germinal disc, and the rest of 

 the yolk serves in a secondary or more remote manner to furnish 

 materials for nourishment of the embryo an<l its accessory parts. 

 Thus we distinguish the germinal from the nutritive or food-yolk, 

 or, as the younger Van Beneden has named them, the protoplasm 

 and the dciiloplasm. 



In the smaller ovum of the mammal, on the other hand, it seem.s 

 as if the whole, or nearly the whole, of the yolk were proto- 

 plasmic or germinal. There may be some admixture of yolk- 

 granules ; but there is not the marked separation or limitation 

 of the protoplasmic substance which is so distinct in birds, and 

 the earliest changes of development extend to the whole com- 

 ponent substance of the yolk, or, in other words, the yolk is 

 entirely germinal. Hence some have given the names of 

 meicblastic and hoh<hlastic, meaning partially and entirely 

 germinal, to these two contrasting forms of ova. There are 

 many of the invertebrate animals of which the ova present the 

 satrre entirely germinal arrangement as in those of mammals, and 

 the antpliio.Mis may be included in the same group. 



The amphibia stand in some measure between the two ex- 

 tremes ; the purely protoplasmic or germinal part occupying one 

 side, and the nutritive or vitelline the other. But among the 

 invertebrates the gradations are often such as to make it difficult 

 to determine under which group the ova should be placed. 



The genesis or formation of the ovum itself, if it be considered 

 with reference to its first origin, carries us back to a very early 

 period of the formation of the parent in which it is produced ; 

 and it is one of the most interesting problems to determine n*iat 

 is the source of the cells in the parent from which the ova 

 originally spring. All that I can venture to say at present in 

 regard to this point is, that the primordial ova or germs appear 

 in the parental body while still embryonic, at a very early period 

 of its development, and clearly derive their origin from a deeply- 

 seated part of the formative cells which are undergoing trans- 

 formation into the primitive organs ; but the exact seat of the 

 origin of the reproductive cells is still a matter of doubt. 



When the ovum attains its full maturity in the ovary, the seat 

 of its formation within the parent, it is separated from that 

 organ, and when perfected proceeds to undergo embryonic deve- 

 lopment ; a marked difference in this respect existing between 

 the germinal product of the higher plants and animals. 



The period of maturation of the ovum is marked in the greater 

 number of animals by a series of phenomena which have gene- 

 rally been interpreted as the extrusion or absorption of the ger- 

 minal vesicle ; and various observers have aetually traced the 

 steps of the process by which that vesicle appears to leave the 

 yolk and is lost to sight, or has passed into the space betwene 

 the yolk and its membrane in the shape of the peculiar hyaline 

 bodies named the polar or directing globules. But recent re- 

 searches, afterwards to be referred to, tend to show that some 

 part at least of the substance of the germinal vesicle remains to 

 form, when combined with the fertilising elemetrt, the newly- 

 endowed basis of future development. 



Among the earliest changes to which the perfect animal ovum 

 is subject, I have first to refer to the segmentation of the germ, 

 a series of phenomena the observation of which has been pro- 

 ductive of most important results in leading to a comprehension 

 of the intimate nature of the formative process, and which is of 

 the deepest interest both in a morphological and histological 

 point oi view. This process, which was first distinctly observed 

 by Prevost and Dumas more than fifty years ago, and is now 

 known to occur in all animal ova, consists essentially in the 



p. 332. See also Darwin on Pangenesis, in his work on "Varialions," 

 Sic. (iS6S), vol. ii. p. 374, and the Review by Ray Lankester ( f Haeckel's 

 wolk, " Perigenesis der Blastidule," &c., in NATUiiE for 1876, p. 235, an 

 Ray I.ankeslcr's Essay on " Comparative Longevity," 1870. 



