14. BECENT LITEBATURE OF THE OVUM. 



and female elements, and every one of its descendants must also contain a certain proportion 

 of each. For the sexual conjugation of two cells it is assumed to be necessary that the one 

 should get rid of the male elements, and retain only the female, and that the other should be 

 exclusively male. This is effected in the one case, according to Minot, by the extrusion of the 

 polar globules, which, in this view, represent the male element of the originally hermaphrodite 

 generative cell, so that when they are extruded this remains wholly female ; in the other case 

 there is also a separation, and the separated part becomes disintegrated, leaving only the male 

 portion, or spermatozoon the separated part in this case represents, therefore, the female 

 element of the generative cell. 



Theory of Weismann. Minot's theory was adopted by Balfour, who looked upon the 

 formation of polar cells as having' been acquired by the ovum for the express purpose of pre- 

 venting parthenogenesis. According to this view no polar globules should be formed in parthe- 

 iiogenetic ova, and it was believed by both Minot and Balfour that they would not be found 

 to occur. It has, however, since been discovered by Weismann and Blochmann that parthe- 

 nogenetic ova do extrude one polar globule, although the ordinary ova of the same animal 

 extrude two. 



It is clear that this fact renders a modification necessary in the view advocated by Minot 

 and Balfour. Such modification, or substitute, as it may perhaps more appropriately be 

 termed, has been furnished by Weismann in his theory of heredity ( Vererbungstheorie). This 

 theory assumes that every animal and vegetable cell contains two different kinds of living 

 substance. These are termed by Weismann the nuclear plasma and the nutritive plasma. The 

 former is endowed, with germinative, directing and hereditary functions, the latter with 

 assimilation of food and the more purely physical functions (contraction, nerve-conduction, 

 secretion, &c.), but these functions are assumed to be carried out under the direction of the 

 nuclear plasma. The nuclear plasma is further supposed by Weismann to consist of two sub- 

 stances, viz., a germinal plasma which is the primitive form, and which alone is endowed with 

 heredity, and a histogenetic plasma which has been derived from the germinal plasma, and which 

 controls the division, growth, and differentiation of the cell. Fertilization consists in the 

 bringing to the ovum of a certain amount of germinal plasma from a different individual, 

 and Weismann assumes that it is necessary for the ovum, prior to fertilization and develop- 

 ment, to get rid both of its old histogenetic plasma and of so much germinal plasma as may 

 be brought to it by the spermatozoon, and that it effects this by the extrusion (1) of one 

 (histogenetic) polar globule, (2) of the other (germinal) globule. If this is what happens, the 

 primitive or germinal plasma is never wholly eliminated from the ovum, so that it may be 

 looked upon as transmitting all the accumulated ancestral characters which have been derived 

 from the vast number of its predecessors. A portion is, however, got rid of in the form of 

 the second polar globule, and what remains is not necessarily of quite the same constitution in 

 every case, nor is the portion of germ plasma brought by the spermatozoon necessarily always 

 similar : these differences in the germinal plasma of the fertilized ovum may account, accord- 

 ing to Weismann, for the individual differences which occur in the progeny. 1 



Weismann and Ischikawa have shown that in some animals the segmentation of the ovum 

 may have advanced through one or two stages before the entry of a spermatozoon. In this 

 case the spermatozoon (male pro-nucleus) blends with the nucleus of only one of the cells 

 which have resulted from the segmentation. Probably the sexual cells are the ultimate result 

 of this conjugation. 



RECENT LITERATURE. 



Beneden, E. v., Recherches sur la maturation de I'ceuf et la fecondation. Arch, de biolog., iv., 

 1884 ; Fertilization and Segmentation in Ascaris Megalocephala, Journal of Microscopic Science, 1888 

 (Bulletin de 1'academie r. des sciences de Belgique, 1887, t. xiv.) ; Sur la fecondation chez I'ascaride 

 megalocephale, Anatomischer Anzeiger. Jahrg., iii., 1888. 



Beneden, E. v. et Neyt, A., Nouvelles recherches sur la fecondation et la division mitosique 

 chez I'ascaride megalocephale (Bullet, de 1'acad. royale des sciences de Belgique, 3 ser. t. xiv., 1887). 



Blochmann, P., Ueber die Richtungskorper lei Insekteneiern, Morphol. Jahrbuch, Bd. xii., 1887. 

 Also in Morph. Jahrb. xv., 1889. and Verhandl. d. naturhist. med. Vereins zu Heidelberg. 1888. 



Boveri, T., Zellenstudien, H. 1, Die Bildung der Richstungskorperlei Ascaris megalocephala und 

 Ascaris lumbricoides, Jenaische Zeitschr. f. Naturwiss., Bd. xiv., 1887 ; Zellenstudien. H. 2. Die 

 Befruchtung und Theilung des Eies von Ascari^ megalocephala, Jena. Zeitschr., 1888 ; Zellenstudien. 

 H. 3. Ueber das Verhalten der chromatischen Kernsulstanz lei der Bildung der Richtungskorper u. 

 lei der Befruchtung, Jena. Zeitschr., 1890. 



Biitschli, O., Gedanken uler die morphologische Bedeutung der sogenannten Richtungskorper chen, 

 Biolog. Centralbl., Bd. iv., 1884. 



1 It is difficult to do any justice to Weismann's theory in a short space, and the above is to be taken 

 as only furnishing a rough sketch of its general outline. For a complete account the reader is 

 referred to "Weismann's publications upon the subject (see Literature). 



