REPRODUCTION. I03 



*' produced zooids " are capable of giving rise to fresh 

 Hydrai by a true generative process. It must also be borne 

 in mind that this production of fresh zooids by a proceSk of 

 gemmation is not so essentially different to the true sexual 

 process of reproduction as might at first sight appear, since 

 the ovum itself may be regarded merely as a highly-spe- 

 cialised bud. In the Hydra, in fact, where the ovum is 

 produced as an external process of the wall of the body, 

 this likeness is extremely striking. The ovarian bud, how- 

 ever, differs from the true gemmce or buds in its inability to 

 develop itself into an independent organism, unless pre- 

 viously brought into contact with another special generative 

 element. The only exceptions to this statement are in the 

 rare cases of true " parthenogenesis," to be subsequently 

 alluded to. 



b. Reproductio7i by Internal "i^cmmation. — Before consider- 

 ing the phenomena of " alternate generations," it will be as 

 well to glance for a moment at a peculiar form of gemma- 

 tion exhibited by some of the Polyzoa, which is in some 

 respects intermediate between ordinary discontinuous gem- 

 mation and alternation of generations. These organisms 

 are nearly allied to the sea-mat, already spoken of, and, 

 like it, can reproduce themselves by continuous gemmation 

 (forming colonies), by a true sexual process, and rarely by 

 fission. In addition to all these methods they can repro- 

 duce themselves by the formation of peculiar internal buds, 

 which are called " statoblasts." These buds are developed 

 upon a peculiar cord, which crosses the body-cavity, and is 

 attached at one end to the fundus of the stomach. When 

 mature they drop off from this cord, and lie loose in the 

 cavity of the body, whence they are liberated on the 

 death of the parent organism. When thus liberated, 

 the statoblast, after a longer or shorter period, ruptures 

 and gives exit to a young Polyzoon, which has essentially 

 the same structure as the adult. It is, however, simple, 

 and has to undergo a process of continuous gemmation 



