HARDWICKE'S SCI ENCE-GOSSIF. 



20I 



and spermatozoa are essentially of the nature of cells, 

 — germ-cells and sperm-cells. It is with the ovum 

 ■or germ-cell that we have here principally to deal, 

 with its structure and the changes previous to, 

 -during, and immediately succeeding fecundation, of 

 which it is the seat. The nucleus of a cell is repre- 

 sented by its germinal vesicle, the nucleolus by its 

 germinal spot, and the protoplasm by its vitellus. 

 P'ecundation is accomplished by the union of a 

 ^perm-cell with a germ-cell. 



Previous to fecundation a very remarkable phe- 

 nomenon occurs in the ovum, which was first noticed 



rig. 123. — Follicle of the hermaphrodite gland of Helix hoy- 

 tenuis. CO, ova ; ss, spermatoblasts (cells producing sperma- 

 tozoa). 



in 1837 by Dumortier in Limnaa stagnalis, but was 

 not thoroughly investigated until eleven years later, 

 when Fritz Muller again brought it before the 

 attention of scientific men. This phenomenon is 

 now known as the extrusion of the polar bodies. It 

 gives evidence that the ovum is approaching a state 

 of maturity, not that it has already approached a 

 -State of ripeness, since Calberla has noticed extrusion 

 ■of polar bodies previous to the egg attaining a 

 matured state. The course of affairs is this. A 



o"o'C)-"ooooo<..ooooO ^ 



(Fig. 124. — Extrusion of a polar body from the egg of Siucinea 

 PJeiff'eri. a, polar body showing elongated filaments of 

 niiclear matter; b, radiated appearance around the polar 

 tody; c, yolk. 



spindle-shaped body is seen passing outwards from 

 •the germinal vesicle to the peripheral portion of the 

 ovum; this body has been named the " amphiaster 

 of rejection " by Fol, and the " direction spindle " by 

 iBiitschli. Fari passu with this the membrane 

 slimiting the germinal vesicle becomes dissolved away 

 in the vitellus, and the yolk around it becomes dis- 

 posed in a radiating manner ; while a little later a 

 -small mass of protoplasm is extruded from the ovum 

 .at the peripheral end of the spindle to form the first 

 polar body— a process which is repeated again. 

 The question as to what this "direction-spindle"' 



is made up has met with some discussion and has not 

 yet been laid down to rest. The most probable 

 explanation seems to be that of Ilertwig — in which 

 he is supported by iJlockmann and Trinchese — who 

 considers it to be fragments of the limiting mem- 

 brane of the germinal vesicle and the germinal spot. 

 Fol thought that it might be due to a swelling of 

 some filaments which he has termed " filaments 

 bipolairs," and which he conceives as passing from 

 the germinal vesicle to each pole of the egg, while 

 Biitschli considered it due to a coalescence of 

 granules merely. 



It is a question whether this phenomenon is one 

 of indirect cell-division or karyokinesis, and this takes 

 us back to discuss over the intricate subjects of the 

 striictures of the nucleus and the nucleolus of the cell 

 and the various changes it undergoes during indirect 

 cell-division. To this consideration we must give 

 some little space. 



In the seventy-seventh volume of Virchow's 

 " Archives " is a paper by Professor Flemming, of 

 Keil, and from the publication of this paper is to be 

 dated the sober beginnings of our present knowledge 

 of nuclear-structure. In a resting nucleus Flemming 

 finds that there exists four determinable portions — a 

 network of fine fibres to which he has given the 

 name of "karyomiton," one 'or more nucleoli, an 

 intermediate or nuclear fluid, and a limiting mem- 

 brane enclosing them all. The fibres of the karyo- 

 miton are often nodular, and show a great affinity for 

 colouring matters, and hence he has named the 

 substance which composes them "chromatin." The 

 nucleoli are to be regarded as a specific product of 

 the reticulum of fibres and as devoted to a special 

 function ; there are differences between the nucleoli 

 present in any given nucleus : some show the presence 

 in them of minute vacuoles, and with some a slow 

 kind of movement obtains. He distinguishes in the 

 limiting membrane an achromatic and a chromatic 

 portion, the former not being stained, the latter 

 being stained by colouring reagents. The nuclear- 

 fluid consists of a solution of albumen and salts. 

 The chromatic portion of the limiting membrane is 

 seen in some cases to be perforated, and through 

 these perforations Klein and Frommann have con- 

 ceived that a communication may probably take 

 place between the reticulum of the fibres in the 

 protoplasm of the cell (kytomiton) and that within 

 the nucleus (karyomiton). 



However, all observers do not agree with Flem- 

 ming as to his description of the structure of the 

 nucleus. The workers on this subject are divided 

 into two schools — those who do, and those who 

 do not, agree with Flemming. Van Beneden, 

 Retzius, and Leydig belong to the former ; to the 

 latter belong Carnoy, Balbiani, and Strasburger. 

 The latter hold not with the former, in that they 

 consider the fibrous appearance of the nucleus to 

 be due to a coiled-up filament, and in that the 



