ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 865 



in the case of animals, L. Guignard lias undertaken a fresh series of 

 observations, principally on the mother-cells of pollen-grains in both 

 monocotyledons and dicotyledons, the embryo-sac, endosperm-cells, 

 parenchyma of ovules, wall of the ovary, &c. 



Eeagents capable of differentiating the constituent elements of the 

 nucleus and of the cell-protoplasm show that the nucleus, when in a 

 state of rest, is composed, within its enveloping membrane, of a 

 hyaloplasm which serves as a substratum for granulations or micro- 

 somes which present the reaction of nuclei, and which are disposed 

 in the form of a network or of a more or less anastomosing branched 

 filament, with one or more nucleoli in the course of, or simply in 

 contact with, the filament. The following phases may be distinguished 

 in the division of the nucleus : — 



1. The chromatic filament existing in a state of rest in the mother- 

 nucleus, or proceeding from a modification of the network, com- 

 mences to contract and to become thicker, assuming the form of a 

 ball. 



2. It then divides into segments, the number of which varies ac- 

 cording to the species and according to the organ of the plant, but 

 seems to be uniform for the same organ. This is the phase of 

 segmentation. 



3. The separate segments behave in different ways in different 

 cases before arranging themselves in the centre of the cell after the 

 disappearance of the membrane of the nucleus. Sometimes they take 

 the form of straight rods arranged in rays ; sometimes they are bent 

 in the middle, turning their angle towards the centre of the cell and 

 their extremities towards the periphery. This is the phase of the 

 nuclear plate of Strasburger, or of the nuclear star of Flemming, at 

 the end of which the achromatic threads of the nuclear spindle 

 usually make their appearance. 



4. In each rod or segment a longitudinal division then takes 

 place (not transverse, as has been stated in the case of plants), 

 analogous to that which several zoologists describe in the case of 

 Batrachians, and which consequently doubles the original number of 

 the segments. 



5. Each half-segment, before taking part in the formation of the 

 two daughter-nuclei, turns one of its more or less bent extremities, or 

 the angle formed by its two branches if there is a curvature in the 

 middle, in the direction of the poles which constitute two new centres 

 of attraction, round which the divided segments arrange themselves 

 radially. 



6. After a contraction at the poles, the segments coalesce by 

 their extremities, in order again to form a filament, the curves of 

 which contract and assume the globular disposition of the mother- 

 nucleus. 



Although Strasburger has described cases in which there is no 

 longitudinal division of the elements of the nuclear plate, Guignard 

 considers that the occurrence of such a division in all the cases 

 observed by him establishes a fresh point of agreement between 

 the vital phenomena in plants and in animals. 



