FROM EGG TO INSECT SNODGRASS 



377 



elsewhere it appears to be reduced to a mere network of strands 

 in the yolk. Near the center of the egg, or usually nearer one 

 end, in an island of protoplasm, is the nucleus {Nu)^ a small body, 

 but one of great importance and of complicated structure. The soft 

 pai'ts of the egg are all contained in a delicate vitelline niembi'wne 

 {Vlt), just within the chorion, which is a product of the egg itself. 

 The egg nucleus consists of a nucleus plasm separated from the 

 surrounding egg plasm and yolk by a nuclear membrane. Within 

 the plasm of the nucleus is a fine network upon which, during 



Fig. 3. — Diagram of cell division, involving special activities in tbe nucleus 

 A, cell in ordinary condition with chromatin (Cm) distributed througli nucleus 

 (Nu), and centrosome (Cen) undivided; B, cell preparing to divide, chromatin 

 in spireme tlireads (/S'pt), centrosome divided and halves moving to opposite poles 

 of nucleus, nuclear membrane disappearing ; C, spireme threads condensed to 

 chromosomes, centro.somes at opposite poles of nucleus with spindle between 

 them, nuclear wall gone ; D, chromosomes in plane between the centrosomes, each 

 splitting into two ; E, chromosomes divided and half of each drawn to opposite 

 ends of nucleus, cell dividing ; P, chromatin dispersed in two new nuclei, and 

 cell completely divided. 



ordinary periods of inactivit}' (fig. 3 A), are scatered many small 

 grains of a dark-staining substance called chromatin {Cm). 



The ^f^g, and the sperm are cells, and the most important activity 

 of cells in general is that of division, a process by which a single 

 cell becomes two cells; and upon this depends the power of groAvth 

 and reproduction. The usual method of cell division involves in- 

 teresting changes in the nucleus, and must be understood before we 

 can proceed with the subject of development. 



When a cell is preparing to divide, the chromatin granules of its 

 nucleus arrange themselves in a number of long thread-like strand« 

 (fig. 3 B, fS/n), or in a single strand tluit later breaks up into pieces. 



