ONTOGENESIS 173 



individuals have been produced asexually, the latter 

 generations produce eggs and sperm. The individuals 

 developing from the zygosis of these gametes again 

 reproduce asexually, and so on. It is the same with 

 the majority of those forms that normally reproduce 

 by budding, fin a colonial hydroid, for example, 

 the individuals of the colony (hydranths) come into 

 existence by the process of budding from other 

 hydranths of the colony, and the majority resemble 

 their immediate progenitors. But some of the 

 individuals thus budded off differ from the rest both 

 in structure and function. Instead of remaining 

 attached to the parent stem, they break away as 

 free individuals, and their peculiarly differentiated 

 structure enables them to live an active existence in 

 the water. These are the hydromedusse. In the 

 second place these individuals, unlike the fixed 

 members of the colony, produce eggs and sperm- 

 cells that are released when mature and conjugate in 

 the open sea. From the zygote thus formed there 

 develops an individual that is unlike the free-swim- 

 ming, sexual medusa, its immediate ancestor, but 

 resembles the hydranth from which the medusa 

 budded off. By repeated asexual reproduction this 

 produces another hydroid colony. In some fresh- 

 water hydras there has been described the develop- 

 ment of sexual organs on buds still attached to the 

 parent stem. If the habit of always developing them 

 in such a fashion should become confirmed, it is easy 

 to see how the condition found in the colonial hydroids 

 may have come about. If some of the buds remained 



