340 ANIMAL BIOLOGY. [Part II. 



spot. In this case the separated portion acquires a new foot, 

 and the basal portion acquires the circumoral tentacles. It is 

 somewhat doubtful, however, whether this process occurs under 

 natural conditions. But there is no doubt that if a hydra be 

 artificially divided, each separated part develops into a new and 

 perfect individual. 



(3.) Sexual Reproduction. In its sexual reproductive organs the 

 hydra differs very markedly from any form we have as yet 

 studied. It is hermaphrodite and self-fertilising; but the testes 

 and ovaries are variable in number, and not definite in position 

 The testes, or spermaries, of which there may be as many as 

 four or five in the same individual, arise as thickenings of the 

 ectoderm from the active local growth of interstitial cells. The 

 neuro-muscular cells here take the form of a thin external layer. 

 Within the testes, which assume a mammiform appearance 

 (Fig. 99, ts.), swarms of spermatozoa may be seen under a high 

 power. Each has an oval head and vibratile tail (Fig. 79, G,), and 

 is developed from one out of the many clear spherical bodies 

 into which the spermatospores derived from the interstitial 

 cells subdivide. 



The ovary (Fig. 100, B., ovy.} is larger than the testis. There 

 may be one or two in the same individual. It, too, originates 

 in the ectoderm, and from the interstitial cells which undergo 

 special differentiation over an area which embraces about half 

 the circumference of the body, and is often, but not always, 

 situated nearer the foot than the testes (Fig. 99, B.). As de- 

 velopment of the ovary proceeds one central cell increases 

 enormously at the expense of the others. This single ovum 

 (Fig. 100, H.) is capable of amoeboid movements, pushing forth 

 pseudopodial processes at various parts of its surface. It con- 

 tains a number of yolk-granules, and is provided with a well- 

 marked germinal vesicle containing a germinal spot. In the 

 green hydra chlorophyll corpuscles are developed within the 

 ovum. Before impregnation by the spermatozoa, the germinal 

 vesicle and its contained germinal spot are said to disappear. 

 The ovum escapes by the bursting of the thin ovarian walls, but 

 still remains attached to the shrunken remnant of the ovary 



