74 ZOOLOGY 



asexual. It usually occurs when the adult size of the animal 

 is attained. It is not confined to the Protozoa or single-celled 

 animals, but may occur in several Invertebrate groups in which 

 (Hydra, Fig. 81) there is not a high degree of specialization. 

 The budded individual or offspring may in such cases consist 

 of one cell or of many. In addition to the internal stimulus af- 

 forded by the attainment of normal size, external conditions 

 such as diminished food supply, temperature changes, etc., in- 

 fluence the process of non-sexual reproduction. 



101. Sexual Reproduction. It seems for some reason that 

 even in the one-celled animals the method of reproduction by 

 division cannot be continued indefinitely without some ill effects 

 to the organism. In many Protozoa there is at certain times a 

 union of two individuals, either temporarily or permanently, 

 accompanied by exchange of nuclear material or by a fusion of the 

 whole protoplasm. After a period of rest and the coming of fa- 

 vorable conditions of life, division begins again with renewed 

 activity. Something similar is seen in the more complex animals 

 the Metazoa. After a period of cell divisions, by which the 

 individual body is built up, the majority of cells, as muscle or 

 nerve cells, appear to lose their power of dividing, and even the 

 less differentiated cells which we have described as the ova and 

 sperm, which arise from the primordial germ cells ( 49), are in- 

 capable of continuing the division necessary to produce a new 

 individual until they have been stimulated by union with each 

 other (or by some artificial means). Such unions of cells, to 

 form by later divisions a new individual, are called conjugation 

 or fertilization, and the new individual which results is said to 

 arise by sexual reproduction. The uniting cells may be similar 

 (as in Pandorina), in which case the union is isogamous. More 

 usually the cells are different and the union is heterogamous. 

 In the latter case the cells are called ovum and sperm (Chapter 

 IV) and are usually formed in different individuals, though 

 very often the same individual may give rise to both classes of 

 cells (hermaphroditism) from different regions of the germinative 

 epithelium, or in the same organ at different times. The 

 special organs in which the ova are produced are called ovaries. 



