220 



ANIMALS 



The bud resembles the parent polyp in form and may be half 

 as large. With the opening of a mouth in the centre of the 

 circle of tentacles the animal is complete. Up to this time 

 the gastro-vascular cavity of the bud has been in communica- 

 tion with that of the parent, but the base of the bud becomes 

 gradually more constricted until finally the bud is cut off 

 entirely and is then an independent organism. Several such 

 buds may be in process of development at one time and by this 

 means the number of individuals rapidly grows. 





^Mo^o?^'; 





Fig. 12 2. — The egg cell of hydra, in amoeboid 

 form. (After Kleinenberg.) 



Fig. 123. — A hydra embryo. 

 The first four tentacles just 

 beginning to develop. (After 

 Kleinenberg.) 



500. Less frequently another type of protuberance may be 

 observed on the column of the hydra. Just below the circle of 

 tentacles may be found a conical eminence which affects only 

 the ectoderm. Lower down on the column, frequently on the 

 same individual, a somewhat similar, though more rounded, 

 protuberance may be found. These are the gonads. The 

 upper ones are testes and in them are developed the sperm cells, 

 which are very small and provided with a flagellum. These are 

 produced in large numbers. The lower gonads are the ovaries 

 and contain finally a single large cell, the ovum. Both ova 

 and sperm cells are derived from the ectoderm, but they recede 

 from the surface and are covered by the ectodermal epithelium 

 during the period of development. When the egg is mature it 



