640 SCIENCE PROGRESS 



In the cytoplasm of cells, excepting those of the higher 

 plants, a pair of minute bodies known as centrosomes is found. 

 These bodies play an important part of which I shall have to 

 speak presently in cell division. They also appear frequently 

 to be connected with the motile appendages of cells. Besides 

 these there is a group of structures in the cytoplasm known as 

 chondriosomes, which are further subdivided according to their 

 structure and appearance. 1 At present, however, we need only 

 consider them as a single group. Apart from certain phenomena 

 connected with chondriosomes which I shall deal with later, it 

 is only necessary to say that they give rise to the fundamental 

 material from which are formed the specific cytoplasmic sub- 

 stances found in the cells of various tissues, such as certain 

 parts of the striped muscle fibres and "prickles" in the cells of 

 the skin. 2 



Cells, as far as we know, have but one mode of origin and 

 that is from preexisting cells. The way in which cells divide is 

 by no means simple. The chromatin in the nucleus becomes 

 collected into a number of well-defined bodies, generally in the 

 form of U's and V's, which are known as chromosomes ; these 

 bodies divide individually, splitting lengthwise, thus ensuring a 

 division which is both quantitative and qualitative. While this is 

 happening, radiations appear in the cytoplasm around the cen- 

 trosomes, some of the radiations running between the two. The 

 centrosomes separate further and further apart, until they are 

 found at opposite poles of the cell with a spindle of radiations 

 extending between them. The nuclear membrane breaks up 

 and disappears, each chromosome becoming attached to one of 

 the spindle fibres. At the same time the cytoplasm takes an 

 hour-glass shape and the half of each chromosome travels 

 towards the opposite poles of the cell, so that when the con- 

 striction in the middle of the hour-glass terminates in the 

 separation of the cell into two daughter cells, an exact repre- 

 sentative half of every original chromosome is present in each. 



1 Meves, F., " Die Chondriosomen als Trager erblicher Anlagen. Cytologische 

 Studien am Hiihnerembryo," Arch. f. Mikro. Anat., Bd. 72, 1908. 



2 Duesberg, J., " Les Chondriosomes des cellules embryonnaires du poulet, et 

 leur role dans la genese des myofibrilles," Arch. f. Zellforschung., Bd. iv. 1910 ; 

 Arnold, G., " On the Condition of Epidermal Fibrils in Epithelioma," Quart. 

 Journ. Micro. Science., vol. 57, part 3, Feb. 1912 ; Firket, J., " Recherches sur la 



genese des fibrilles epidermiques chez le poulet," Ana/. Anz., Bd. xxxviii. 191 1. 



