SEXUAL REPRODUCTION 1G5 



proceeds to regenerate the lost portion. vSome worms also 

 throw out buds or lateral branches which remain in contact, 

 and thus, as in Syllis ramosa, a marine worm which lives 

 inside a sponge, a regular branching net-work is i'ornu d. 



The so-called gemmules of sponges are formed in the late 

 autumn in temperate climates, and in tropical climates at the 

 beginning of the dry season. Each gcmmule is built up of 

 a number of ordinary cells which secrete around them a 

 capsule, the whole resembling a seed. They are capable of 

 withstanding most adverse conditions and can survive severe 

 frosts and the drying up of the water in which they liv'c. They 

 are more common among fresh-water sponges than among 

 marine forms. 



The statohlasts of the Polyzoa, fairly highly organized 

 animals living in colonies, resemble in general the gemmules 

 of sponges. They again consist of a mass of cells surrounded 

 by a hard capsule and their object in life is to keep the race 

 going w^hen their parent forms have died down owing to frost 

 or drought. The capsules are very characteristic of the species 

 to which they belong. In temperate climates they are formed 

 in the autumn and germinate in the warm spring days. 



Sexual Reproduction 



We have seen that the simplest form of reproduction is 

 dividing into two. When the two products are of equal size, 

 as is the case with Amoeba, the process is caWed fission, but 

 when they are of unequal size, as in the yeast-cells, the process 

 is called gemmation and for some illogical reason the larger 

 product is regarded as the mother. A stimulus to this fission 

 is often given by the fusion of two organisms. These may be 

 of similar size, as in the case of Paramoecium. Two Para- 

 moecia will lie close to one another and interchange some of 

 their protoplasm. This is known as conjugation and is often 

 a prelude to a fission. In other unicellular plants and animals 

 sexual spores may be formed, that is to say, the protoplasm 

 divides into 2, or 4, or 8, or more, separate individuals usually 

 in a capsule, from which they escape and then may fuse two 

 by two. A further stage is when the spores are of unccjual 



