CELL DIVISION. 



31 



is completed when the young nuclei, proceeding from the two poles 

 of the nuclear spindle and the surrounding clear protoplasm, have 

 attained their definite size, and the remains of the fibres have been 

 absorbed. 



During these processes the protoplasm of the cell has gradually 

 become more and more constricted by a furrow which is directed 

 transversely to the long axis of the nuclear spindle, and which after 

 the completion of the division of the nucleus brings about a separa- 

 tion of the cell contents into two masses the daughter cells 

 (fig. 18). 



If the products of the division are unequal, so that the smaller 

 portion may be looked upon as a production of the larger, we give 

 the name " budding " to this form of reproduction. 



FIG. 18. Processes of cell division in an embryonic blood corpuscle of a chick (after 

 Biitschli). -5T, nuclear spindle. Ep, nuclear plate or equatorial thickening. 



Finally, the term endogenous cell formation is applied to that 

 method of increase in which the cells originate within the mother- 

 cell. In this case the protoplasm does not divide by a progressive 

 constriction and separation into two or more parts, but differentiates 

 itself round the newly formed nuclei, with which the original nucleus 

 may persist. 



The ovum which we have to contemplate as the starting-point of 

 the development of the organism produces by these various methods 

 of cell multiplication the material of cells which serves for the for- 

 mation of the tissues. Groups of originally indifferent and similar 

 cells break up and assume severally a changed appearance. The 

 constituent elements undergo various differentiations, and from them 

 and their derivates is produced a definite form of tissue, endowed 

 with a function corresponding to the peculiarity of its structure. 



The separation of groups of different cells leading to the establish- 

 ment of various tissues prepares the way for the physiological 

 division of labour between the organs, which, like the tissues compos- 

 ing them, can, according to the functions which they perform, be 

 divided into organs of vegetative life and orglins of animal life. 



The former have to do with the nutrition and maintenance of 



