ALTERNATION OF GENERATIONS. 



187 



process or chain-making. Sexual individuals are budded off from 

 the asexual, into which their fertilized ova in turn develop. This 

 must, of course, be distinguished from cases where asexual multipli- 

 cation 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. 



Fig. 63. — The alternation of generations in the common Jellyfish A urclia; 1, the free-swimming emhryo, 

 orplanula: 2, the embryo settled down; 3, 4, 5, 6, the developing asexual stage, or hydratuba; 7, 

 8, the formation of a pile of individuals; 9, the liberation of these; 10, 11, the aquisition of the free- 

 living sexual medusa form. — From Haeckel. 



From a fertilized ovum in Salpa, a nurse or asexual individual 

 develops. This has a rootdike 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 fertilized 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 

 fertilized 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 individuals, 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 



