THE HYDRA AS A SIMPLE METAZOAX 273 



number of spermatozoa must perish, as the ova are produced in 

 much smaller numbers and fertilization consists in the union of a 

 single spermatozoon and a single ovum. 



Like the spermatozoa, the ova arise from interstitial cells 

 (Fig. 132 B). In the young ovary there are several dozen germ 

 cells, the oocytes, each formed from an interstitial cell. As the 

 ovary develops, one of these cells outstrips the rest and begins 

 to ingest its fellows much as an amoeba might devour other 

 amoebas. Indeed, the ovum is actually amoeboid in shape during 

 this stage of its development (Fig. 132 A). The process of ingestion 

 continues until there remains only a single egg cell of large size. 

 The ovary then consists of this one egg covered by a thin layer of 

 ectoderm which ruptures at the time of maturity and exposes the 

 surface of the ovum to the water. In this manner the spermatozoa 

 have access to the ovum at the time of fertilization. Following 

 fertilization, the Z3'gote undergoes di\asion and, after secreting 

 about itself a C3'st-like membrane, the early embryo drops to the 

 bottom at a stage of development which differs in the various 

 species of hydra. Development now proceeds slowly or rajiidly, 

 according to the season, until an outer layer of cells, the ectoderm, 

 and a solid inner mass, the endoderm, are formed. Tentacles 

 then develojj and the embryo bi-eaks from its cyst, elongates, 

 becomes attached at one end, develops within the endoderm a 

 cleft which forms the enteron, forms a mouth at the free end, and 

 so becomes a young hydra. When the zygotes are formed in 

 late fall, the developing individual passes the winter in the egg 

 shell at the bottom. 



The life cycle of hydra, therefore, is as follows: Two germ cells, 

 or gametes, unite to form a single cell, the zygote, which by cell 

 division and differentiation becomes the adult animal with its 

 many-celled organization. From this adult, masses of cells may 

 be detached to form new individuals by budding; or single cells, 

 the germ cells or gametes, may unite in fertihzation for the pro- 

 duction of a new individual after the manner described. By 

 comparison with the ceU cj'cles previously described (Fig. 110, p. 

 215), it will be seen that, save for the budding, such a cycle is typical 

 of all metazoa. The general comparison between cell cycles in pro- 

 tozoa and metazoa will be recalled in this connection. 



Regeneration. — Related to the process of budding is the phe- 

 nomenon known as regeneration, or the formation of new indi- 



