REPRODUCTION OF HYDRA 95 



develop into a complete Hydra. This power of regeneration, or 

 re-growth of the whole animal from a part, is a characteristic 

 of all of the lower animals and is an evidence of their generalized 

 character. Some forms of the lower animals (certain worms 

 for example) actually reproduce by spontaneously breaking into 

 pieces. Hydra does not reproduce in this way but, in addition 

 to sexual reproduction, has a more simple method of propagating 

 its kind, namely, by budding. At certain times, on healthy 

 well-nourished Hydras, swellings appear near the foot. Such a 

 swelling, which involves both layers ectoderm and endoderm, 

 soon takes the form of a young Hydra, mouth and tentacles 

 appearing on the distal end and the trunk growing until it has 

 nearly the length of the parent organism. The bud or young 

 Hydra then separates from the parent by constriction of the 

 foot and starts on an independent career (Fig. 5). 



Among the cells of Hydra, distributed here and there in the 

 tissues, are male and female germ cells which collect from time 

 to time in definite regions to form the gonads. Of these the 

 testes or male organs are formed near the base of the tentacles, 

 while the female organ or ovary is formed nearer the pedal 

 disc. 



The spermatozoa are developed from the formative or inter- 

 stitial cells of the ectoderm, and collect in large numbers be- 

 tween the epithelio-muscle cells. They do not change directly 

 into spermatozoa, but each one is a primordial sperm cell which 

 divides two or more times before transforming into sperma- 

 tozoa. When mature the spermatozoa are liberated by rupture 

 of the outer walls of the testis, and live for a longer or shorter 

 period in the water until they come in contact with egg cells; 

 otherwise, they die. 



There is usually only a single ovary, and in the ovary only a 

 single egg. But the female gonad, like the testes, starts with 

 an accumulation of formative cells which divide and produce a 

 number of potential egg cells. Only one develops, however, the 

 others being devoured by the successful egg, which, as an amoe- 

 boid cell, puts out pseudopodia and feeds upon the sister cells 

 (Fig. 38, A). The ovum grows to a relatively large size and be- 



