Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/436

418 REPRODUCTION [AMI MAX. one exception confirmatory, penetrate somewhat further into details. The general process can be made out with especial clearness in fig. 13. B c D Fio. 13 (after Fol). A, spermatozoa in mucilaginous coat of ovum of Astcrias ylacialif, a prominence rising from surface of ovum towards a spermatozoon; in B they have all but met, and in C they have met ; D, spermatozoon enters ovum through distinct opening ; H, ovum showing polar vesicles, and approach of male and female pronuclel; E, K, G, later stages in coalescence of the two nuclei. Oscar Hertwig had clearly enunciated in 1875 that " fertilization depends upon the conjugation of two sexually differentiated nuclei." This view has, however, lately (1883) been controverted by A. Schneider, who regards the radii which are seen in the ovum about the pronuclei as arising from the substance of the germinal vesicle alone, and who maintains the origin of the male pronucleus independently of the spermatozoon, which he believes to disappear. As Van Beneden, Nussbaum, and Flemming have since, however, successively controverted these views, Hertwig's account of the process may safely be retained, at any rate with the slight modi- fication insisted on by Flemming, who prefers to describe the con- jugation-nucleus as arising from the union of "the chromatin of a male with that of a female nuclear body. " In Van Beneden's work on the fecundation of Ascaris the penetration of a single spermatozoon into the vitellus and its transmutation into the male pronucleus are copiously figured and described. Most important, however, is perhaps his account of the formation and segmentation of the conjugation-nucleus. The. female pronucleus undergoes changes resembling those preceding ordinary nuclear division ; so also does the male ; and the chromatin of each breaks up from the state of a single continuous and convoluted fibril into two V-shaped loops. The two pro- nuclear membranes now fuse ; and the resultant conjugation-nucleus thus contains four loops of chromatin, of which two are male and two female. Each loop now undergoes longitudinal division ; a nuclear spindle is meanwhile forming from the achromatin substance of the segmentation-nucleus, and this now proceeds to divide, but in such wise that each of the two daughter nuclei receives two of its four half loops of chromatin from the female and two from the male pronucleus. A circular furrow appears, dividing the ovum equatorially ; a cell plate like that of a vegetable cell arises in the same plane ; and the two first blastomeres are soon completely formed, and proceed to redivide in the same way. The results of Nussbaum are essentially similar so far as they go ; Strasburger's recent work summarized below contains a thorough confirmation of these accounts of the process of fecunda- tion so far as plants are concerned ; while most recently (1885) 0. Hertwig has re-stated his original theorem with a discussion of its physiological aspects and consequences. (14) Segmentation of the Ovum. The process of segmenta- tion has already been repeatedly mentioned. As with the other histological problems presented by the ovum, our knowledge of its minute details is in a somewhat unsettled yet rapidly advancing state, due to the pro- gress of the wider inquiry into cell structure and cell division in general. The essential correspondence of the changes to be observed in the conjugation-nucleus and yolk with those of ordinary cell-division (see PROTOZOA, vol. xix. p. 833, fig. 1) has been worked out by many FIG. 14. Regular segmentation of ovum (after Gegenbaur). observers, and Van Beneden's more minute account of the process is summarized above. Our knowledge of its external features is of much older date, and has been com- paratively recently brought together by Balfour (Embryo- logy, i., chap, iii.); the present limits permit only the briefest summary. In the simplest case that of small ova destitute of food-yolk, the ovum divides by a vertical plane into two segments or blastomeres, each of these again into two, and so on, and a mass of 4, 8, 16, 32, G4, &c., cells is thus formed, but in the resultant mulberry-mass or morula the cells are all equal and similar. Such segmentation is termed regular ; commonly, however, the regularity and equality of ordinary cell-division is more or less interfered with by the presence of food-yolk. In this regard Balfour enunciates the valuable general law " that the velocity of segmentation in any part of the ovum is, roughly speak- ing, proportional to the concentration of the protoplasm there, and that the size of the segments is inversely pro- portional to the concentration of the protoplasm." Bear- ing this in mind, the varieties of segmentation are intelli- gible enough ; thus the unequal segmentation of the Frog's ovum (fig. 15) needs no further explanation. These two FIG. 15. Unequal segmentation of Frog's egg. (From Balfour after Ecker.) preceding types, in which segmentation is complete, were distinguished by Remak as " holoblastic," and were long looked upon as wholly distinct from " meroblastic " ova like that of Fowls or most Fishes, in which segmentation is partial, i.e., confined to a small area of the surface of the yolk. Balfour's law, how- ever, explains these cases as only an ex- flo-trpratinn nf tliP nrP Fl - 1<5 (after Balfour). A, section of developing ' ovum of an Elasmobrancli, showing meroblastic Ceding Case, due to segmentation; B, optical section of centroleci- , thai Crustacean ovum. the greater accumula- tion of food-yolk, and the consequent check to segmenta- tion at the lower or vegetative pole. Another form of partial segmentation is presented by many Arthropods, in which the yolk occupies a central position within the ovum, and three varieties of this " centrolecithal " process are dis- tinguished equal, unequal, and superficial. While the ontogenetic process of segmentation in general and of regular segmentation in particular is usu- ally regarded as being a recapitulation of the phylogenetic development of the primitive Metazoon from its Protozoan ancestor, the vast nutritive importance of the food-yolk, and the consequent high variability of its quantity in relation to the habits and circumstances both of parent and offspring, render its wide variations in detail a totally unsafe guide to affinities. A single genus may, in fact, exhibit extreme forms. (11) 3. SYSTEMATIC AND GENERAL EMBRYOLOGY. While it is impossible to compress within the limits of the present article the subject-matter of a full treatise on embryology, an attempt must be made to touch in turn upon (1) the empirical facts, (2) the concrete inductions, and (3) the abstract generalizations of development. 1. A sketch of the history of this branch of the science is given under EMBRYOLOGY (q.v.), while for detailed infor- mation as to the division and differentiation of the ovum in each group, and as to the more or less marked changes by which the resulting embryonic forms assume the adult organization, the reader must be referred to the separate articles (SPONGES, HYDROZOA, MOLLUSCA, &c.), as well as to the classical work of Balfour. The more important empirical facts need thus only the briefest outline. Protozoa. Since the young Protozoan, whatever its mode of origin from the parent form may be, rarely differs from the latter