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and so we have produced a tiny Hydra growing out from the body of 

 the parent with which its layers and cavity are directly continuous. 

 When conditions are favourable, more than one bud may be formed ; 

 indeed, specimens are sometimes met with in which the buds them- 

 selves have budded, so that temporary associations are formed. 

 Sooner or later, however, the daughter individuals drop off and lead 

 an independent life. Hydra exhibits another phenomenon some- 

 what allied to this production of buds, and although it can hardly 

 be considered as an ordinary means of reproduction, doubtless plays 

 its part in cases of injury. If an individual be cut into a number of 

 pieces, provided they are not too small, and contain fairly repre- 

 sentative parts of both ectoderm and entoderm, each piece is capable 

 of regrowing into a complete animal. This is a power we term 

 Regeneration, which is fairly common among lowly organised beings, 

 but becomes more and more limited as we ascend the animal scale. 

 Sexual reproduction also occurs as a normal method in Hydra, 

 but no alternation or definite relation between it and the asexual 

 has been shown to exist, and it is the more uncommon of the two. 

 The testes at first consist of a mass of interstitial cells which at a 

 certain stage are termed the spermatogonia. These divide, producing 

 two spermatocytes, and each of these in turn gives rise to four sperms. 

 The adult spermatozoa are composed of a small oval head to which 

 is attached a fairly long vibratile tail. They are aggregated in 

 large numbers between the epithelio-muscular cells of the ectoderm, 

 and so form noticeable colourless swellings, the testes. When quite 

 ripe the ectoderm cells split, releasing the free-swimming sperms into 

 the water. Like the testes, the ovaries also commence as a collection 

 of intestitial cells which form the oogonia, but only one of these 

 is destined to become the sex cell. This divides up, and one of the 

 daughter cells, i.e. the obcytes, so produced assumes a central 

 position in the mass. It sends out pseudopodia and feeds upon its 

 sister cells much in the same way as an Amceba ingests its food. 

 The result of this feeding is that the cell grows to a relatively 

 enormous size, and the products of digestion are stored up within 

 it in the form of numerous tiny spherical masses, the yolk spheres or 

 deutoplasts. This amoeboid stage is not at all common in developing 

 ova among animals, and when it is ended the oocyte withdraws its 

 pseudopodia and rounds off. It then undergoes two successive 

 unequal divisions, giving off two tiny masses of protoplasm, each 

 with a certain amount of nuclear material, the first and second polar 

 bodies, and so becomes a ripe ovum ready for fertilisation. By this 

 time it has been surrounded by a thin gelatinous layer, and increased 

 so much in size that it has caused the ectoderm'cells to split, exposing 

 part of its surface to the water ready for the sperm. Fertilisation 



