544 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



and posteriorly into a fresli head ; several zones of 

 gemmation may appear before the zooids break away 

 from the parent, and begin to develop generative 

 organs. In Protula, the parent reproduces sexually, 

 as well as the buds, but in Autolytus the genital 

 glands are confined to the zooids that have been de- 

 veloped by budding. 



The most complicated alternations are found among 

 the Urochordata,* a large number of which multi- 

 ply by budding ; a simple case is presented by those 

 forms in which the bud arises as an outgrowth of the 

 body wall, together with a prolongation of part of the 

 intestine. From this outgrowth the organs of the 

 bud are fashioned, and the bud, breaking away, gives 

 rise to fresh buds. Both bud and parent develop 

 generative organs and reproduce themselves sexually. 



In Botryllus the product of a fertilised ovum gives 

 rise to a single bud ; this gives rise to two, each of 

 which again develops two buds ; the four buds ar- 

 range themselves round a common cloaca, then give 

 rise to two or three buds, and these again to others. 

 These last, which may go on budding, are the first that 

 are provided with sexual organs. 



In Pyrosoma the product of a fertilised ovum gives 

 rise, while still an embryo, to four zooids ; these re- 

 produce sexually, and so give rise to fresh colonies, or 

 multiply by budding, and so increase the size of the 

 colony. 



The height of complexity is reached by Doliolum, 

 the embryo of which is at first tailed, but becomes 

 cask-shaped in form, like its parent. From its dorsal 

 surface there grows out a process or Stolon, at the 

 sides and along the dorsal middle line of which buds 

 appear. The former become converted into the spoon- 

 like forms of Gegenbaur, and become free ; their 



* The account given by Balfour (" Comparative Embryology," 

 voL ii.) has been closely followed here. 



