THE STRUCTURE OF CELLS 



alveolar theory see in the fibrils a honeycomb structure, the cavities 

 are generally admitted to be reduced to the vanishing point. 



Strasburger has attempted to utilise both the filar and the 

 alveolar hypothesis, considering that in every cell the protoplasm 

 outside the nucleus consists of two distinct parts. The one, which is 

 specially nutritive and alveolar, he terms Trophoplasm ; the other, 

 which is more closely concerned with the dynamical changes in the 

 cell, and possesses a filar structure, he designates as Kinoplasm. The 

 relations of the kinoplasm will be more specially considered in 

 connection with nuclear division. 



Besides the protoplasm and nucleus, there are present other 

 organised structures in the cell. The vacuoles, which have long 

 been recognised, are cavities in the protoplasm, and lined apparently 

 with a specialised layer of this substance. In some cases they are 

 rhythmically contractile as in many of the Protozoa. 



But it is around that enigmatical body, the centrosome, that 

 especial interest has persistently attached ever since its first definite 

 discovery by Van Beneden in 1885. The centrosomes are minute 

 granules, most often situated either singly or in pairs in each cell, 

 and in close proximity to the nucleus. They are frequently con- 

 tained in a specialised mass of protoplasm, termed by Boveri the 

 Archoplasm. 



Centrosomes and their attendant structures have been differ- 

 ently described by various observers. Van Beneden, to whom we 

 owe the first recognition of these bodies, distinguished, in the case 

 of Ascaris, a central granule, surrounded by two concentric areas, to 

 which he gave the names of medullary 

 and cortical zones respectively. Boveri 

 described in the cells of the same 

 animal a centrosome surrounded by a 

 lighter zone, from which it was definitely 

 cut off by a kind of limiting membrane. 

 Within the centrosome he further dis- 

 tinguished a central granule, the cen- 

 triole. The latter body divides before 

 the centrosome increases by fission. 



In still other cases (e.g. in cells of the 

 testis of Salamander) various observers 

 (Meves, Driiner, etc.) have distinguished FIG. 4. 



a whole Series Of Concentric zones Ascaris megalocepkctla. Schematic 



i ,v T x v figure of diaster stage of the first 



around tne CentrOSOme. In the giant cleavage mitosis, c, centrosome ; m.z, 



cells of the spinal cord and in leuco- ggj^g, fffiei"' c rtical Z ne ' 

 cytes, Heidenhain has distinguished a 



group of granules, which replace the single or paired one 

 more commonly met with, and in other cases again there is a 

 reticulate sphere (echinoderms) containing a varying number of 



