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A TEXTBOOK OF PHYSIOLOGY 



The reproductive cells may be of two kinds asexual and sexual. 

 The former occur only in the lower forms of life. The simplest 

 reproduction is by the dividing of the parent organism or cell into 

 two daughter cells. This is accompanied by the changes in the 

 nucleus, either amitotic or mitotic. Sea-stars may shed a whole 

 "finger, which will then develop in.to a new individual like the parent. 

 A fragment of the stem of a hydroid suspended in water will generate 

 a head at each end (Fig. 465). 



Other organisms, such as the polyps, reproduce by a process of 

 budding. From the parent organism a bud springs out, which gradu- 

 ally develops into an adult organism comparable in every way to the 

 parent. Such a new organism may remain attached to the parent, 

 thereby forming a colony of cells, or it may become detached, and 

 lead an altogether independent existence. The nucleated portion 

 (Fig. 466) possesses the property of regenerating an entire individual, 

 but not the non -nucleated portions. 



FIG. 468. 



A , Early stage in meiotic division ; B, the pairs of chromosomes separating in meiotic 

 division. (C. E. Walker.) 



But the commonest form of reproduction is that pertaining to the 

 higher organisms namely, sexual reproduction. In this form of 

 reproduction the essential act consists in the actual union of two 

 different types of cell the gametes (the male and the female cell) 

 often from two separate individuals. The male cell is termed the 

 spermatozoon. It consists of little else than nuclear material. It 

 varies in shape, and is small in size. It is essentially active, its function 

 being to seek out the female cell and fertilize it. The female element, 

 on the other hand, is essentially passive. Often it is large in size, 

 containing a large amount of reserve food material, at the expense of 

 which the fertilized cell develops. 



These reproductive elements arise from undifferentiated germ 

 epithelium. At first, multiplication of the cells is by means of the 

 karyokinetic division already described, the cells thus formed being 

 termed respectively spermatogonia and oogonia. In the final stages, 

 however, a new form of cell division appears, known as meiotic 

 division, or heterotype mitosis. In this form of division but half 

 the number of chromosomes is formed, as compared with the usual 



