Mr. W. Garstang. On the 



[May 19, 



very large dimensions, the number of ectodermic processes has in- 

 creased to 36, and each of the lateral buds has given origin to a pair 

 of buds of the second generation. At this stage there are five rows 

 of stigmata on each side ; but whether the fifth row is formed by the 

 subdivision of a transverse primary stigma, or not, I have been unable 

 to determine. 



This is the most advanced stage in the progressive development of 

 the oozooid of Botryllus that I have seen, and there is reason to 

 believe that the number of rows of stigmata does not increase after 

 this point. In a young colony, very little older than the one just 

 described, the oozooid has undergone great reduction in size, and is 

 evidently dying away, while the two buds of the first generation, to 

 which it gave rise, have grown considerably in size, and are almost 

 fully organised. 



It is perfectly clear from the above account that in Botryllus the 

 stigmata of the full-grown oozooid are secondary formations, due to 

 the subdivision of a series of transversely elongated primary stigmata 

 with which the larva is provided. It is also noteworthy that the 

 primary stigmata — if an inference may be drawn from their relative 

 sizes — arise one after another in regular order from before backwards, 

 and that they are subsequently subdivided in the same order. 



It will be convenient to distinguish the transversely elongated 

 primary stigmata by some distinctive name, and on this account 

 I piopose for them the term " protostigmata." It cannot be denied 

 that these structures present striking analogies with the true gill- 

 clefts of Amphioxus and the lower Vertebrata. 



Turning now to the buds of Botryllus, a remarkable difference is to 

 be observed in the mode of origin of the stigmata, a difference which 

 has important bearings upon the question of their phylogenetic 

 history. Transverse protostigmata are never formed in the buds, 

 whatever be the number of the generation to which the buds belong. 

 The stigmata of the four first rows arise almost simultaneously as 

 small rounded perforations which are entirely independent of one 

 another ; they soon begin to elongate in an antero- posterior direction 

 and rapidly assume their definitive form. 



Botryllus, therefore, exhibits both the modes of development which 

 are known to occur in Ascidians ; the stigmata in the oozooid 

 arise by the subdivision of protostigmata, and the stigmata of the 

 buds appear quite independently of one another. This fact renders it 

 possible to determine which method is the more primitive. In any 

 contrast of this sort between larval and bud development, there can 

 be no doubt that it is the larva which exhibits the primitive mode, 

 while the development in the bud is secondary and modified. 



Now the development of the stigmata in the oozooids of Clavelina 

 and Distajolia proceeds in essentially the same manner as in the buds 



