INTRODUCTION. 



REPRODUCTION. 



A study of reproduction logically precedes that of Embry- 

 ology. Reproduction essentially consists in the separation of a 

 portion of an organism which has the capacity of developing into 

 a form similar to that which gave it origin. The simplest 

 modes of reproduction are those which occur amongst the 

 Protozoa. 



In this group, reproduction may take place in a great variety 

 of ways. These may be classified in three groups: (i) fission, 

 (2) budding or gemmation, (3) spore formation. 



Reproduction in all these ways may take place either subse- 

 quently to and apparently in consequence of a very important 

 process known as conjugation, which consists in the temporary 

 or permanent fusion of two or more individuals, or spontane- 

 ously, i.e. independently of any such previous conjugation. 



Reproduction by fission consists simply in the division of the 

 organism into two similar parts, the nucleus when present 

 becoming divided simultaneously with the cell body. This 

 mode of reproduction is the simplest conceivable, and is not 

 followed by a development, since the two organisms produced 

 are exactly similar, except in size, to the parent form. Besides 

 single fission, a process of multiple fission may take place, as 

 amongst the Flagellata, where Drysdale and Dallinger have 

 shewn that an individual enclosed within a structureless cyst 

 may divide first into two, then into four, and so on. 



The process of budding differs mainly from that of simple 

 fission in the fact that the two organisms produced are dissimilar 

 in size, and also that the separation of the smaller organism 

 from the larger is preceded by a process of growth in the latter, 

 so that in the separation of the bud no essential part of the 

 parent form is removed. This mode of reproduction is found 

 amongst the Infusoria, Acineta, &c. An interesting variation in 

 it is the internal gemmation of many of the Acineta, where a 

 portion of the internal protoplasm with part of the nucleus is 

 separated off to form a fresh individual. This mode of gemma- 

 tion is connected by a series of gradations with the normal 



