Heredity and the Origin of Variations. 129 



But there is a third method of multiplication in hydra : 

 this is the sexual mode of reproduction, and occurs 

 generally in the autumn. On the body-wall of certain 

 individuals, near the tentacles, conical swellings appear. 

 Within these swellings are great numbers of minute sperms, 

 with small oval heads and active, thread-like tails. They 

 appear to originate from the interstitial cells of the outer 

 layer (see p. 124). Nearer the foot, or base of attachment, 

 and generally, but not quite always, in separate individuals, 

 there are other larger swellings, different in appearapce, 

 of which there is generally only one in the same individual 

 at the same time. Each contains a single ovum, or egg- 

 cell, surrounded by a capsule. It, too, and the cells which 

 surround it would appear to be developed from the inter- 

 stitial cells. It grows rapidly at the expense of the sur- 

 rounding tissue, but when mature, it bursts through the 

 enveloping capsule, and is freely exposed. A sperm-cell, 

 which seems, in some cases at least, to be produced by the 

 same individual, now unites with it ; the egg-cell then 

 begins to undergo division, becomes detached, falls to the 

 bottom, and develops into a young hydra. 



Here, then, we have that sexual mode of reproduction 

 which occurs in all the higher animals. It is, however, in 

 some respects peculiar in hydra. In the first place, the 

 ovum is nearly always in other animals (but occasionally 

 not in hydra) fertilized by the sperm from a separate and 

 distinct individual. In the second place, the germinal cells 

 are generally produced, not from the outer layer, but from 

 the middle layer, which appears between the two primitive 

 layers. In some allies of hydra, however, they take their 

 origin in the inner layer ; and it has been suggested that, 

 even in hydra, the true germinal cells may migrate from 

 the inner to the outer layer. But of this there does not 

 seem to be at present sufficient evidence. In any case, 

 however, the essential fact to bear in mind is that a new 

 individual is produced by the union of a single cell pro- 

 duced by one organism and of another cell produced in 

 most cases (but not always in the hydra) from a different 



K 



