August 2, 1888] 



AATURE 



129 



ON PARTIAL IMPREGNATION} 



TOURING our researches on the formation of polar-bodies 

 (see NATURE, vol. xxxvi. p. 607) we made the following 

 observations, which are of considerable interest in connection 

 with the theory of sexual reproduction. 



As we were able to show that parthenogenetic eggs form only 

 one polar-body, while sexual eggs give rise to two, we looked 

 cut principally for those cases in which both kinds of eggs are 

 present in the same species. 



On examining the sexual eggs (" Dauereier") of certain 

 species of Moina, we found, to our astonishment, that even 

 those which possessed a firm vitelline membrane, and in which 

 four segmental cells were already present, still contained a 

 sperm-cell. 



We first of all took this to be a supernumerary spermatozoon 

 which had penetrated into the egg, but it was soon apparent 

 that all egg^ of a corresponding stage contained a similar sperm- 

 cell, and that there was always one only. Further observations 

 showed us that we had had here to do with a case of partial 

 impregnation. Only one of the first four segmental eel s, and not 

 the entire egg-cell, becomes united -with the sperm-cell. This is 

 the case, at least, in Moina faradoxa. In Moina rectirostris, 

 impregnation must occur at a rather later stage, for in this 

 species we have seen eggs in which the first four segmental cells 

 were again ready for division, and still the sperm-cell had not 

 fused with one of them. 



In Moina faradoxa the process takes place as follows: — Im- 

 mediately after the extrusion of the egg into the brood-chamoer, 

 it is a naked sausage-shaped mass. Jn this stage, a spermato- 

 zoon penetrates into it in the region of the vegetative pole, and 

 then the vitelline membrane becomes formed, and prevents the 

 entrance of a second. The germinal vesicle at the same time 

 becomes transformed into the first polar-spindle, which lies at 

 the surface ; the first, and soon afterwards the second polar- 

 body then becomes constricted off, and the nucleus of the ovum, 

 surrounded by protoplasmic particles, migrates to the centre of 

 the egg. which has by this time contracted to the usual form. 

 Now follows the first division of the ovum, which, however, 

 only consists in a se paration of these first, or, as we will call them, 

 secondary egg-cells in the centre of the egg ; — the two first seg- 

 mental cells come to lie, as usual, in its longitudinal axis — one, 

 which is always recognizable by the proximity of the polar- 

 bodies, nearing the animal pole, the other the vegetative pole. 

 The sperm-cell always lies in the neighbourhood of the latter, 

 without, however, yet becoming united with it. 



Then follows a second division of the segmental cells, toge- 

 ther with the separation of the daughter-cells in the transverse 

 direction. There are now four star-shaped daughter-cells pre- 

 sent, which lie at an almost equal distance apart, at a right 

 angle with one another. The sperm-cell can be seen near one 

 of the two lower (hi/item) cells, and it now begins to show 

 amoeboid movements, and to approach the segmental cell, a 

 short narrow bridge of protoplasm being formed, and the two 

 cells beginning to unite with one another. Fusion then follows, 

 and in the next following stage, of eight segmental cells, no sperm- 

 tell can any longer be see// 11/ the 



The uniting of the sperm-cell with the cell- and nuclear- 

 constituents of the egg thus only takes place after the embryonic 

 development has already advanced to the four-celled stage. It 

 would naturally be of great interest to know what eventually 

 becomes of those segments which are concerned in fertilization 

 ■ — that is, which parts of the embryo are formed from them. A 

 very possible supposition is, that only those parts of the egg 

 become fertilized out of which the germ-cells of the young 

 animal will subsequently be formed. This conjecture is rendered 

 by no means improbable by the fact that it is one of the two 

 segmental cells lying at the vegetative pole of the ovum which 



1 Translated from a paper by A. Weismann and C. Ischikawa (BtrickU 

 iter Naturforschenden Gesellschaft z/t Freiburg '7 />> -> l*d- lv -> Heft i, 

 p. 51) — W. N. P. 



becomes fertilized ; for it is from these cells, according to Grob- 

 ben*s beautiful discovery with regard to the summer eggs of 

 Moina, that the germ-cells arise. At a future time we hope to 

 be able to speak more definitely on this point : at present it is 

 only necessary to add that we are studying these processes in 

 other Daphnidce, and have already observed a similar series of 

 stages in Sida crystallina to those above described. But in 

 this case fertilization occurs earlier, in the two-celled stage of 

 segmentation. 



Freiburg i/B., December 12, 1887. 



P. S. — In the continuation of the above observations an- 

 other case has presented itself, in which impregnation does 

 not take place until eight segmental cells have been formed. This 

 happens in Daphnia p/ilex. Further details concerning partial 

 impregnation, as well as theoretical support of the facts treated 

 of above, we reserve for a future occasion. 



May 21, 1888. 



Addendum to the above Note on Partial I/z/p/rg/zalion, 1 by 

 Weismann and Ischikawa. 



Since giving a short abstract of the observations which 

 led us to the conclusion of the existence of partial im- 

 pregnation, we have continued our researches, and have 

 come to the conclusion tint, in spite of the entire accuracy 

 of our facts, we were mistaken as to the explanation of the 

 phenomena described. The fusion with one of the eight first 

 segmental cells does indeed take place regularly, but the uniting 

 cell is not the sperm- cell. The first segmentation nucleus is 

 here, as in all sexual eggs, formed by the fusion of the nucleus of 

 the ovum with the sperm-nucleus, and the fusion of the two 

 cells observed by us at a later stage is something additional to the 

 ordinary impregnation. That this is the case is quite certain : 

 we found the sperm-nucleus and its subsequent fusion with the 

 egg-cell to occur in the same ova in which we could prove the 

 presence of that cell which we at first took to be the sperm-cell. 



We can hardly be blamed for this error if it be borne in mind 

 that we found this cell, without exception, in every egg which had 

 just passed into the brood-chamber ; that the vitelline membrane 

 was formed directly afterwards ; and that, on the other hand, a 

 fusion of this cell with one of the first eight segmental cells lying 

 at the vegetative pole of the egg could be seen in all ova which 

 we possessed of this stage, viz. in five species— two species of 

 Moina, two of Daphnia, and one of Polyphemus. The fact 

 that the form and size of the supposed sperm- cell differ from 

 those of the sperm-cells in the testis of the corresponding species 

 was indeed an objection to our explanation : it has, in fact, 

 almost the same size and shape in all species. But the sperm- 

 cells become altered as soon as they pass into the egg, and it 

 was shown some time ago by Fol and Hertwig, and more 

 recently by Boveri, that the sperm nucleus grows considerably 

 when within the ovum. Moreover, in one of the species 

 examined {Polyphemus), as well as in Bythotrephcs, the sperm- 

 cell is extraordinarily large, and in both these species we 

 followed the entrance of the tnormous amceboid sperm-cell into 

 the ovum by means of sections, step by step, and were able to 

 convince ourselves of its essential correspondence with the 

 supposed sperm-cell in the eggs of o her species. What else 

 could this cell within the ovum be, if it were not the sperm-cell? 

 It was never wanting, and on the other hand there was always 

 one only, so that any idea of its being a parasitic organism was 

 out of the question. Moreover, the two polar cells were always 

 present, so that it could not be mistaken for one of these. And 

 up to the present time no one had ever seen any other cell but 

 the sperm -cell within the ovum. 



We should hardly, indeed, have discovered our error so soon, 

 if we had not remembered that one of us had found some years 

 ago that unimpregnated sexual eggs of Daphnidie soon become 

 disintegrated, > and had we not asked ourselves how the embryonic 

 development advanced in such unimpregnated eggs before dis- 

 integration begins. For, as we believed that the sperm-cell was 

 only ready for conjugation in impregnated eggs after they had 

 segmented into eight parts, it was to be expected that segmenta- 

 tion would take place up to this stage in unimpregnated ova, and 

 that then only would the disintegration begin. Had we found 



1 Translated from the proof of a paper to appear in the Berichte der 

 Naturforsch. Geitllschaft »u Freiburg HB., Bd. iv. Heft 2, 1888.— W.N. 1'. 



■ See Weismann, " Beitrage zur Naturgeschicht- der Daphnoiden,' iv. ; 

 "Ueberden Einfluss der Begattung aufdie Erzeiiiung von Wintereiern," 

 Zeitscli f. U'/'ss. Zool., Bd. xxviii. p. iq8 et seq. 



