Dec, 8, 1887] 



NA TURH 



i'3'5 



Certain recent observations on the maturation of the 

 ovum are of great interest in this connection, as illustrat- 

 ing the possible mechanism by which ovogenous plasma 

 in the extrusion of the first polar body, and a number of 

 ancestral plasmata in that of the second, are removed 

 from the nucleus of the ovum ; the former process 

 being designed to equalize in bulk the ovogenous and 

 germinal plasmata contained in the nucleus {Aequations- 

 thcilung), the latter to reduce the total number of an- 

 cestral plasmata present by a half {Reduktions-theilung). 



For this reduction in number of the ancestral plasmata 

 there must be a second and special form of karyokinesis 

 not as yet generally recognized. If any value be attached 

 to the fact, first observed by Flemming,^ that in normal 

 karyokinesis the nuclear loops are split longitudinally, 

 one of the resultant halves passing to each daughter- 

 nucleus, then the two nuclei produced by such division 

 must be precisely alike, not only quantitatively, but 

 quahtatively. For Prof Weismann's view, however, there 

 must exist "a type of karyokinesis in which the primary 

 equatorial loops are not split up, but separated into two 

 groups,each of which groups forms one of the two daughter- 

 nuclei." E. Van Beneden- has already shown that in the 

 formation of the polar body of Ascaris tnegalocephala the 

 nuclear division differs from the usual type of karyo- 

 kinesis in that the plane of division is at right angles to 

 the normal ; and Carnoy ^ has more recently essentially 

 confirmed the observation, and has further added that, 

 of the eight nuclear loops which are to be found in the 

 equator of the spindle, four are removed in the extrusion 

 of the first polar body, and two of the remaining four with 

 the second. Were it certain that each of the eight loops 

 consisted of ancestral plasma, it would be necessary to 

 regard the first division of the nucleus as a process of 

 reduction, not of equalization ; but this is not to be 

 accepted, mainly because the extrusion of the first polar 

 body is to be found also in parthenogenetic ova. With more 

 probability the first polar body of the ovum <ji Ascaris is 

 to be regarded as removing ovogenous plasma, since we 

 know, through the observations of Flemming and Carnoy, 

 that under certain conditions secondary splitting and 

 consequent numerical duplication of the nuclear loops 

 may occur. This shows, in Prof Weismann's view, that 

 there exist nuclei in which the same ancestral plasmata 

 may be present in different loops. Such " identical loops," 

 however, are not necessarily at the same ontogenetic 

 grade ; and this is probably the case here, as the four 

 loops of the first polar body must be regarded as 

 ovogenous plasma, the other four as germinal plasma. 

 This would be practically proved if it could be shown that 

 the eight loops ,were produced by longitudinal splitting of 

 four primary loops, since such splitting is the means of 

 separating plasmata of different ontogenetic grade from 

 one another, without diminution of the number of ancestral 

 plasmata. 



With regard to the male cell, the facts at our disposal 

 are too few to enable us to speak with such confidence as 

 is the case with the ovum, but whether the theory of Fan- 

 genesis, or of the Continuity of Germ-plasma, be proved 

 correct, a process of reduction of ancestral plasm similar 

 to that occurring to the ovum must also take place in the 

 maturation of the sperm cell, though probably after a 

 different manner. The ancestral plasmata of the ovum 

 undergo reduction only at the termination of ovarial 

 maturation. Supposing, however, that reduction affected 

 the first ovicell of an organism only, and that the rest 

 were produced from this by normal division, then there 

 would be practically but two kinds of ova in the 

 ripe ovary, coresponding to the two halves of the 

 original ovicell, and but two kinds of individuals 

 produced from them, the members of each kind re- 

 sembling each other as closely as twins. On the 



' Arch. Mikr. Anat. xvi., and elsewhere. 



^ Arch. Biol. iv. 3 La Cellule, 1886. 



other hand, the later the period of germ-cell-formation 

 at which the reduction is effected, the more will the 

 ova differ in composition fronT one another, and the 

 greater scope is afforded for variation among the 

 resultant individuals. Finally, if reduction be deferred 

 till the ova be mature, the variation insured among the 

 progeny is as great as it is possible to achieve. The 

 production of such maximum variation is the probable 

 explanation of the fact that the second polar body is not 

 extruded till the end of maturation. With the sperm- 

 cells, however, the conditions of number and size are 

 different from those obtaining in the ova. Though it is 

 quite conceivable that the process of reduction may be 

 deferrei till the completion of sperm-cell formation (both 

 of the fission-products probably remaining as sperm-cells), 

 still the other possibility must also be considered — 

 namely, that it may take place at an earlier date in the 

 formation of the sperm, since the opportunity for extreme 

 variation, however necessary in the case of ova of which 

 a considerable proportion are fertilized, is far less requisite 

 among sperm-cells, of which perhaps one in a hundred 

 thousand or a million may be actually effective. The 

 question can only be settled when we know which of the 

 forms of nuclear division it is that effects the reduction of 

 ancestral plasmata ; in the meantime there is evidence 

 to show that different types of fission are found at 

 different stages of sperm-cell formation. Van Beneden 

 and Julin "^ have shown that direct and karyokinetic 

 division alternate in the spermatogenesis of Ascaris 

 megalocephala ; and the observations of Carnoy ^ and 

 Plattner ^ on Arthropoda further point to the occurrence, at 

 certain stages, of that less-known type of karyokinesis 

 which, according to Prof Weismann, is cha > t .. ' 

 the process of reduction. The " Nebenkern" described 

 by La Valette St. George as occurring at the penultimate 

 stage of spermatogenesis, is probably comparable to the 

 first polar body extruded by the ovum. 



As is now generally known, Prof. Weismann has suc- 

 ceeded in demonstrating that one, and only one, polar 

 body is extruded from the parthenogenetic ovum ; but 

 the memoir* dealing with the details is but recently 

 published, and is in a periodical inaccessible to most 

 readers. His observations cover the following species : — 



OSTRACODA. 



Cypris reptans. 

 Cypris fusca. 



ROTIFERA. 



Callidina bidens. 

 Conochilus volvox. 



Cladocera. 



Polyphemus oculus. 

 Leptodora hyalina. 

 Bythotrephes longimamus. 

 Moina rectirostris. 

 Moitta paradoxa. 

 Daphnia longispina. 

 Daphnella brachyura. 

 Sida crystallina. 



The process in the Cladocera is as follows. The nucleus 

 of the ovum approaches the periphery, and becomes 

 gradually fainter till it is no longer recognizable except 

 by the help of reagents. A normal nuclear spindle is then 

 formed, and the polar body cut off with the resultant half- 

 nucleus. After extrusion, the polar body may in some 

 instances not only segment, and one of the resultant 

 cells again segment, but, in the case at least of Moina, it 

 appears that it secretes that part of the egg-shell \vhich 

 immediately overlies it, so that its true cell-nature is m- 

 disputable. . 



With regard to the Rotifera, a group in which its 

 occurrence has been denied, a true parthenogenesis is 

 proved by the following observation. A female oiCalhdttui 

 with two uterine embryos was isolated on a slide ; after 



' Bull. Acad. Belg. (3) vii. 312. ... / La Cellule, 1885. 



3 /niern. Menafschr./. Anal. Hisfol. in. Htft 10. 



4 Wei-mann and Ischikawa, " Ueber dre Bildung der Richtungskorper 

 bei thierischen Eiern," Ber. Naturf. Gesell., Freiburg i. B., ui. pt. i. 44 

 pages, 4 plates. 



