190 



GENERAL PHYSIOLOGY 



parts that arise, every daughter-cell, is correspondingly smaller ; it 

 can then grow in turn until it has reached the limit of its indi- 

 vidual measure. But in the reproduction of the cell by division, 

 parts must pass over into the daughter-cells from both the essen- 

 tial cell-constituents, the nucleus and the protoplasm, otherwise 

 the daughter-cells would not represent complete cells, and hence 

 could not continue to live. 



In another chapter in which we shall consider the mechanical 

 explanation of vital phenomena we shall have to enquire after the 

 deeper-lying causes of gTowth and of limitation in the size of cells. 

 In this place it is necessary merely to obtain an outlook over the 

 field of vital phenomena. If it be accepted provisionallj^ that 

 reproduction is merely further growth, while the size of the cell is 



limited, it follows that all repro- 

 duction depends upon a division 

 of the living substance of the 

 cell. The widely different varie- 

 ties of reproduction are nothing 

 but cell-division ; and Virchow 

 has rightly extended the old dic- 

 tum of Harvey, " 07nne vivum ex 

 ovo, " into that which forms the 

 basis of all modern ideas of re- 

 production, " omnis cellula e 

 celhda." 



This is at once evident in 

 unicellular organisms. They re- 

 produce simply by the division 

 of their cell-body, each daughter- 

 cell assuming during the division 

 the shape and form of the mother- 

 cell ; and if, as in the Infusoria, 

 the cells possess various kinds of 

 appendages or organoids, the elements that are lacking become 

 regenerated after the division of the body (Fig. 67). But in multi- 

 cellular organisms, both animals and plants, special reproductive 

 organs are developed, the cells of which become constricted off and 

 as eggs develop by repeated cell-division into similar organisms 

 (Fig. 68). In organisms that have separate sexes the sexual cells 

 of the reproductive organs are different in the male and the female 

 individuals. The male sexual cells are the sperm-cells, or sperm- 

 atozoa, the female the egg-cells, or ora. For the production of a new 

 individual a union of the two sexual cells, called fertilisation, 

 must take place, except in certain cases where parthenogenesis 

 is present, i.e., where individuals capable of life can develop from 

 unfertilised eggs, as with many Crustacea and insects. Finally, 

 in the lower multicellular animals, in addition to sexual repro- 



FlG. 69. — Myrianida, a worm in the process of 

 - fission. The single individuals are still 

 hanging together like the links of a chain. 

 a, The original animal ; b, c, d, e, J, g, the 

 buds from the oldest (6) to the youngest 

 (g). (After Milne-Edwards.) 



