ALTERNATION OF GENERATIONS. 203 



ever into a jelly-fish, but into a sessile hydroid-like organism or 

 "hydra-tuba." By growth and division this asexually produces 

 the jelly-fish in turn. Here the sexual generation is more stable 

 and conspicuous, the reverse of the former case, but the same 

 formula applies. 



Or take a case from another class of animals, the marine 

 worms. Some of the syllids have the following life-history. A 

 worm remains asexual, never attaining either the external 

 characteristics or the internal organs of the sexual individuals. 

 It gives rise to these, however, by an asexual process of chain- 

 making. Sexual individuals are budded off from the asexual, into 

 which their fertilised ova in turn develop. This must, of course, 

 be distinguished from cases where asexual multiplication is only 

 a phase preceding the acquisition of sexuality. The above 

 cases are again expressible in the simplest formula. 



{b) Now take a more complex case, from among the tunicates, 

 the highest point at which the genuine alternation can be said 

 to occur. From a fertilised ovum in Salpa, a nurse or asexual 

 individual develops. This has a root-like process or stolon, on 

 which buds are formed. These are set free together, and form a 

 chain of sexual salps. The chain finally breaks up. The fertilised 

 ova of the sexual salps grow up into nurses again. Now the 

 only emphatic complication here is the liberation of a chain of 

 individuals at once ; otherwise the formula holds perfectly good. 



In the allied Doliolum, however, the case is different. From 

 a fertilised ovum a nurse, or asexual individual, develops as 

 before. This produces a number of primitive buds, which 

 cluster about the nurse. Many of them form nutritive in- 

 dividuals, and these we may leave alone. But others become 

 " foster-mothers," and go free, carrying with them a few of the 

 primitive buds, — as it were their younger sisters. The foster- 

 mother remains asexual, is a bearer merely, and need not further 

 complicate the series. But the primitive buds which have been 

 carried away give rise asexually to secondary buds ; these become 

 sexual, and their fertilised ova give rise to the original "nurse" 

 forms. There are therefore several asexual generations between 

 the sexual, and our formula must run, — 



