PHYSIOLOGY OF THE INVERTEBRATA. 401 



upon by some scientists as one of the " modes of reproduc- 

 tion." 



Asexual repkoduction includes the processes of gem- 

 mation, fission, endogenous cell formation ; and a variety of 

 asexual reproduction is known by the name of partheno- 

 genesis. 



(rt) Gemmation. — In this mode of reproduction a small 

 portion of the body enlarges and gradually increases in size. 

 When fully developed this bud may either become detached 

 from the parent and develop into a free organism (like the 

 parent), or it may remain permanently attached to it, giving 

 rise to a colony. 



(h) Fission. — This mode of reproduction, common in the 

 lower animals (and of special importance in the formation of 

 new cells), consists simply of a division of the animal into 

 two or more parts. Each part then grows and ultimately 

 assumes the same form as the parent ; and possesses the same 

 power of reproducing its kind. Should the division, how- 

 ever, remain permanently incomplete, colonies of the animal 

 will be produced. 



(c) Endogenous cell formation. — This mode of asexual 

 reproduction or agamogenesis occurs in the Protozoa. The 

 animal becomes encysted — i.e., it surrounds itself with a cover- 

 ing or cell-wall. After this, the nucleus becomes constricted 

 and ultimately may be divided into many portions. The 

 protoplasm then divides in a similar manner, and there may 

 result two, four, eight, &c., cells, in each of which there is at 

 least one nucleus.* These cells finally rupture the parent- 

 cell and are set free. 



(d) Partlicnogcnesis. — As already stated parthenogenesis is 

 a variety of asexual reproduction. In this case the whole 

 development of the embryo is effected without the succour of 

 fecundation. Parthenogenesis is the production of young, 

 apparently without any previous congress with the male 



* Such a process is termed segmentation, and may be seen in the early 

 stages of the development of the embryo of higher forms. 



2 C 



