NtJCLEI OP ANIMAL AND VEGETABLE CELLS. l43 



observer, viz. as vacuoles in the intermediate band of the 

 bistellate figure ; and it may be ascertained that the total 

 mass of the nuclei after division is larger than before. 



Nothing more could be made out by examination of fresh ova. 

 After hardening in -V p. c. solution of perosmic acid for two 

 to five minutes and staining in carmine a further differentiation 

 may be seen. The elongating nucleus is marked by longitu- 

 dinal rods with thickened centres, which together form a me- 

 dian thickened zone. The nucleus, moreover, continues long 

 after the time of its seeming disappearance in the fresh con- 

 dition, extending as a band from the centre of one star to 

 that of the other, and exhibiting at this stage two lateral 

 thickened zones, one midway between the centre and each 

 end. The clear portion of the band situated between them gets 

 longer as cleavage proceeds, and the lateral thickened zones 

 at last become the young nuclei. 



As the result of his observations, Hertwig concludes that 

 the nucleus is an automatic centre, situated within the 

 cell-body and equipped with active forces. These forces de- 

 termine the changes of form which occur immediately after 

 fertilisation, viz. the amoeboid movement and the elongation 

 of the nucleus as well as the formation of the various 

 thickened zones, which must be regargded as due to an 

 aggregation of nuclear substance {Kernsubstanz as dis- 

 tinct from nuclear fluid, Kernsaft) similar to that observed 

 in the growth of nucleoli. By means of other internal forces 

 the nuclei are capable of assimilating to themselves material 

 derived from the general cell-mass — a conclusion rendered 

 necessary by the constant increase in the mass of nuclear 

 matter after division. Other forces, again, acting on the sur- 

 rounding protoplasm, cause a density in the neighbourhood 

 of the nucleus which occasions the radiate arrangement of 

 granules so characteristic of cell-division. 



With regard to the final act in the latter process, viz. the 

 segmentation of the cell-mass, Hertwig considers the forces 

 at work to be proper to the protoplasm and quite indepen- 

 dent of those acting on the nucleus. 



In addition to the above complete researches we have 

 before us a preliminary communication by Van Beneden^ 

 published in the year 1875. 



As no extended research in mammalian embryology had 

 appeared since the time of Bischoff", it seemed to Beneden that 



' ' La Maturation de I'oeuf, la Fecondation et les Premieres Phases du 

 Developpement Embryonnaire des Mainmiferes, d'apres des Recherches 

 faites chea le Lapin.' Bruxelles. 



