Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/434

416 REPRODUCTION [ANIMAL. Flemming and Strasburger, Pfluger and Hertwig; the solution, however, is not complete. Again, can we recognize in the ovum any indication of the position of the future embryo any fixed points, anterior and pos- terior, lateral or even polar further, of course, than the obvious distinction due to the presence of yolk? After the old theory of " evolution " of the embryo, according to which the egg contained the complete organism in miniature, had been finally replaced by that of epigenesis, the wholly undifferentiated form of the ovum seems to have become tacitly assumed. Recent observers, e.g., Van Beneden (see below), have, however, been so far reviving the old view in that they endeavour to distin- guish, even in the unfertilized ovum, the position of the ends and sides of the embryo; others dispute this, and an interesting controversy is in progress. The specula- tion that a more or less considerable share in the differen- tiation of the ovum might be due to the separation of its various constituents according to their different specific gravities at first apparently emitted by Jager, has also reappeared in this regard. Pfluger has observed the segmentation of Frogs' ova fixed in various positions, and describes the plane of first segmentation as constantly vertical, whatever might be its angle to the morphological axis of the ovum, uniting black and white poles. Devel- opment too was usually normal, save that when the upper hemisphere was entirely white abnormality and death followed, and even if inversion was less complete segmenta- tion often stopped. Further experiments led him to the conclusion that the nervous system, and correspondingly other organs, may develop from any portion of the egg- substance that the egg in short is "isotropic." Certain limitations, however, appeared : the blastopore never arose on the upper hemisphere, nor (like the nervous system) ever on the black region, but both always at the intersec- tion of the white area with the third equatorial plane of segmentation. This he terms the point of crystallization of the specialized organism, and goes on to speculate as to the molecular structure of the ovum. These results have, however, been the subject of keen criticism. Thus Roux showed that the specific gravity of the black pole is distinctly less than that of the white, and found that on eliminating the action of gravity by the use of a centrifugal machine the development remained normal, and on repeating Pfliiger's own fundamental observations described the axis of segmentation as coincid- ing with that of the ovum. After similarly eliminating light, heat, and earth-magnetism, he sums .up in precisely opposite terms to Pfluger, who had laid all stress on the influence of the forces of the environment, and regards development as purely a process of self-differentiation. O. Hertwig also maintains that the influence of gravity is only a secondary one, the plane of division being for him determined by the position of the axis of the dividing nucleus, and this again having a definite relation to the form and state of differentiation of the surrounding proto- plasm. He admits, however, the indirect importance of gravity on eggs having a yolk, and so leaves the question still to a considerable extent open. In another paper he adopts Pfliiger's conception of the isotropy of the ovum, holding that the yolk is not so organized that from any definite region of it a definite organ arises, but that the nucleus is the sole centre of activity and control. It is thus evident that a reinvestigation is needed which would embrace the whole question of geotropism. See PHYSIO- LOGY (VEGETABLE). (12) Maturation of the Ovum. Polar Bodies. The period of development and nutrition of the ovum may be regarded as complete when the full size and complex structure above described have been reached, and in this state it most frequently leaves the body of the parent. In the majority of cases at least, new changes have still to be gone through before fertilization takes place, still more development ; and a series of important structural modi- fications, doubtless the expression of extensive functional rearrangements, takes place. To this new and obscure phase of the life-history of the ovum the term maturation has conveniently been applied. Although some of these phenomena have long been familiar to embryologists, the classical investigations are the comparatively recent ones of Blitschli, Oscar Hertwig, and especially Fol (1877) ; more lately those of Sabatier, of Flemming, and above all of Van Beneden. If we postpone details, the main facts of the process as until recently understood can be most readily grasped from Fol's figures of the ovum of the Starfish, of which the most important are copied in fig. 10. In the new-laid egg the germinal FIG. 10. A, ripe ovum of Asterias glacialis, with excentric geiminal vesicle and spot; B-E, gradual metamorphosis of germinal vesicle and spot; F, de- tachment of first polar body and withdrawn! of remaining part of nuclear spindle within the ovum; G, portion of living ovum with first polar body; H, formation of second polar body; I, after formation of do., showing the remaining internal half of the spindle in the form of two clear vesicles; K, ovum with two polar bodies and radial striae round female pronucleus; L, expulsion of polar body. (A-K after Fol ; L after Hertwig.) vesicle is at first a clearly defined sphere, with well- marked membrane, reticulum, and germinal spot. It begins, however, to become irregular and changeful in form, its membrane and reticulum meanwhile disappear- ing, with the apparent dissolution of a portion of its con- tents in the yolk. The germinal spot also vanishes, and two ill-defined clear spots are alone distinguishable in the yolk. Treatment with reagents shows these clear spots to correspond to two star-like figures like those of a certain stage of the division of nucleus of an ordinary cell, while in a somewhat later stage a nuclear spindle is shown near the surface of the egg. The upper portion of this is segmented off (surrounded by very little of the protoplasm of the ovum) as a polar body, while the lower portion of the body remains as the " female pronucleus," and may be seen surrounded by radial striaB. Two or even more polar bodies may be formed, but they never take any part in the subsequent life-history of the ovum, and sooner or later disappear. In view of the profound theoretic importance of this subject, it is necessary to summarize some of the most important recent observations. Thus Balbiani and Weismann (1882) describe the occurrence of polar vesicles in the ova of Insects, in which they had long been supposed not to occur, and Flemming (1885) repre- sents a nuclear figure which he regards as corresponding to at any rate the first stage of the formation of a polar vesicle, in the ova of Mammals, at the period of bursting of the Graafian follicle. Such researches practically complete the chain of evidence for the generalization that the process of maturation of the ovum is essentially similar in all classes of Metazoa, and this not only in its initial stages of internal rearrangement or modification, but even in its later stage of polar vesicle formation. The absence of polar vesicles in parthenogenetic ova (predicted on theoretic grounds) seems, however, possible in some cases, but is by no means certain in all. Our knowledge of the process of maturation has been much extended and modified by the recent elaborate monograph of E. Van Beneden (1883). He finds the egg of Ascaris to show not only two definite poles a " pole of impregnation " and an opposite "neutral pole" but some indication of the anterior and posterior regions and consequently of the right and left sides of the embryo. The substance of the ovum is also remarkably differentiated, that