254 THE CELL 



division of a fertilised egg, remain associated together, forming a 

 colony of cells or an organic individual of a higher order. Re- 

 garded from the common point of view, from which we here treat 

 the sexual question, they may be compared to the collection of 

 cell individuals, multiplying asexually by division, which are 

 derived from a fertilised mother Infusorian. The generation 

 cycle closes here, when in the multicellular organism sexual cells, 

 which have become mature, unite after the processes of fertilisa- 

 tion have occurred, and thus form the starting-point for new 

 generations of dividing cells. The generation cycle may, in this 

 case, present a very different picture, being sometimes very 

 complicated in character. The simplest form is seen in many of 

 the lower multicellular Alga?, such as Eudorinn, or Pundorina. A 

 cell colony (Fig. 137) is produced by the repeated division of the 

 fertilised cell. After having lived for a definite period, all the 

 cells become sexual cells. In order that conjugation may occur, 

 the whole colony produced by cell division splits up into in- 

 dividuals, which serve as starting-points for new generation cycles. 



The capacity, which each cell thus exhibits of reproducing the 

 whole multicellular organism, is not seen when the organism is 

 somewhat more highly developed. The cell substance, which has 

 been derived from a fertilised egg, and which has multiplied by 

 division to an immeasurable extent, then separates itself into two 

 masses, one of which consists of cells which serve to build up the 

 tissues and organs of the plant or animals, and the other of those 

 destined to function in reproduction. In consequence the or- 

 ganism generally remains unaffected in itself when it reaches 

 sexual maturity; it continues to detach the sexual elements from 

 itself, so that they may start new generation cycles, until in con- 

 sequence of the deterioration of the cells of its own body, or from 

 any other cause, it succumbs to death (Nussbaum VII. 33 ; 

 Weismann VII. 48). 



In its purest form, a fixed and definite cycle is only to be met 

 with in the higher animals, in which multiplication of individuals 

 is only possible through sexual reproduction. In many species of 

 the animal and vegetable kingdoms sexual and asexual multipli- 

 cation take place simultaneously. In addition to the cells which 

 require fertilisation, there are others which do not need it, and 

 which, having detached themselves from the organism in the 

 form of spores or pseud-ova, or as large groups of cells (buds, 

 shoots, etc.), give rise to new organisms solely by repeated 



