32 General Part. 



parent and bud; in fission, tlie two individuals are exactly 

 alike. These methods of reproduction are, however, not sharply 

 defined, so that it is often quite impossible to say whether fission or 

 budding has taken place. In the course of the special part, various 

 instances of asexual reproduction will be met with, especially in 

 Ccelentera, Platyhelminths, Chastopoda, and more rarely in 

 Echinoderms, in addition to the Protozoa. 



Often in gemmation or fission the new individual does not separate 

 completely from the other, but remains in more or less intimate 

 connection with it. In this case, if division is repeated, the result is a 

 colony or stock consisting of a varying number of animals, 

 produced by asexual reproduction from one original individual. The 

 members of the colony have lost their independence to an extent 

 corresponding with the closeness of their connection with one another 

 {see special part). Stocks or colonies occur especially in Corals, 

 Hydroids, Tape-worms, Polyzoa, and Tunicates. 



Begeneratiou. — Nearly akin to asexual reproduction is the power of 

 replacing parts of the body wHoh have been lost by some accident, by new 

 formations resulting from growth of the tissue nearest to the wound : the edges 

 of the epidermis form new epidermis; the connective tissue, new connective 

 tissue, and so on. Different animals display this faculty in very different degrees. 

 In MammaUa, for instance, it hardly appears at all ; they can, indeed, repair 

 injvired epidermis and the Uke, but the loss of larger portions of the organism 

 {e.g., tail, hmbs) is not made good. It is more evident in certain lower Vei^- 

 brates, e.g., Lizards, where the tail may be replaced, or in Newts, which can not 

 only form a new tail, but also new limbs. Amongst Invei-tebrates, even in so 

 complex an organism as the Earthworm, large tracts of the body can be 

 regenerated; indeed, in some animals, the power of repair is so gi-eat that, 

 when cut into two or more parts, they grow out into as many new animals ; 

 the best known example of this is the Fresh-water Polyp (Hydra). 



Whilst asexual reproduction occurs in some animals and is 

 wanting in others, all Metazoa exhibit sexual reproduction, which 

 consists essentially in the development of a single cell, after 

 liberation from the parent, into a new individual. Every cell in 

 the Metazoan body has not this capacity, but only certain peculiar 

 ones called ova. As a rule, the ovum cannot develop by itself 

 into a new individual; it must first be fertilised, i.e., it must 

 unite with another cell, usually of a smaller size, and always vvdth 

 special properties, a spermatozoon. 



The ovum is generally rounded, often spherical. Like other 

 cells it consists of protoplasm with a nucleus, the germinal 

 vesicle, which often encloses a large nucleolus (or several), the 

 germinal spot. The ovum is often surrounded with a covering 

 of varying thickness, the vitelline membrane, in which may 

 be one or more openings for the entrance of the spermatozoon. In the 

 protoplasm, there are usually numerous fatty or albuminous particles, 

 the yolk granules, spherules, o r discs, which are all called yolk 

 granules, or in contrast to the protoplasm in which they lie, 

 the deutoplasm. 



