68 



ZOOLOGY. 



the instrumentality of the old is reproduction. In many of 

 the lower animals this is merely a growth process, " growth 

 beyond the limits of the individual." In the single-celled ani- 

 mals reproduction means the formation of the protoplasm into 

 two or more masses, by dividing into two equal parts (divis- 

 ion), by breaking into a large number of sub-equal portions 

 (fragmentation), or by budding (Chapter III, 39). In bud- 

 ding there is the formation of a local outgrowth which ulti- 

 mately attains the size and character of the parent. In division 

 the resulting individuals cannot be distinguished as parent and 

 offspring. Such reproduction, involving only one parent, is 

 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. 79) 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 stimulus afforded 

 by the attainment of normal size, external conditions such as 

 diminished food supply, temperature changes, etc., influence 

 the process of non-sexual reproduction. 



98. 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 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 a fusion of 

 the whole protoplasm. After a period of rest 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 are incapable of con- 

 tinuing 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 



