36 METAGENESIS. 



formed are sexual. The ova come to maturity before the 

 spermatozoa, so that cross fertilization takes place. 



In Botryllus we have clearly a rudimentary form of alter- 

 nations of generations, in that the sexually produced larva is 

 asexual, and, after a series of asexual generations, produced 

 gemmiparously, there appear sexual generations, which however 

 continue to reproduce themselves by budding. 



The type of alternations of generations observable in 

 Botryllus becomes, as pointed out by Huxley, still more marked 

 in Pyrosoma. 



The true product of the ovum is here (vide p. 25) a rudi- 

 mentary individual called by Huxley the Cyathozooid. This 

 gives rise, while still an embryo, by a process equivalent to 

 budding to four fully developed zooids (Ascidiozooids) similar 

 to the parent form, and itself dies away. The four Ascidio- 

 zooids form a fresh colony, and reproduce (i) sexually, whereby 

 fresh colonies are formed, and (2) by ordinary budding, whereby 

 the size of the colony is increased. All the individuals of the 

 colony are sexual. 



The alternation of generations in Pyrosoma widely differs 

 from that in Botryllus in the fact of the Cyathozooid differing 

 so markedly in its anatomical characters from the ordinary 

 zooids. 



In Salpa the process is slightly different 1 . The sexual forms 

 arc noiv incapable of budding, and, although at first a series of 

 sexual individuals are united together in the form of a chain, so 

 as to form a colony like Pyrosoma or Botryllus, yet they are so 

 loosely connected that they separate in the adult state. As in 

 Botryllus, the ova are ripe before the spermatozoa. Each 

 sexual individual gives rise to a single offspring, which, while 

 still in the embryonic condition, buds out a 'stolon' from its 

 right ventral side. This stolon is divided into a series of lateral 

 buds after the solitary asexual Salp has begun to lead an inde- 

 pendent existence. The solitary asexual Salp clearly corre- 

 sponds with the Cyathozooid of Pyrosoma, though it has not, 

 like the Cyathozooid, undergone a retrogressive metamorphosis. 

 By far the most complicated form of alternation of gene- 



1 Vide p. 33. 



