234 THE ASCIDIANS. 



arising simultaneously, and in representing, in all proba- 

 bility, the two halves of a primitive gill-slit, cut in two by 

 a tongue-bar. 



The stigmata which are added to these must, therefore, 

 be regarded as secondary modifications, hardly comparable 

 to the successive formation of new gill-slits in Amphioxus. 



In the Ascidians, therefore, we can only detect the 

 representatives of one pair of primitive gill-slits, and there 

 is every reason for supposing them to be homologous with 

 the first pair of gill-slits in Amphioxus as defined above. 



The six primary stigmata of each side give rise, by re- 

 peated subdivision, to the innumerable stigmata of the 

 adult, both in Ciona and Molgula. The following de- 

 scription, however, applies more particularly to Ciona. 



In the first place, the primary stigmata grow to a sur- 

 prising transverse length, and then commence to divide 

 into two equal portions by small tongue-like projections, 

 which grow across the aperture indifferently from the 

 anterior or posterior walls of the respective stigmata, and, 

 fusing with the opposite wall, divide the transversely 

 elongated slit into two completely separated halves. Then 

 each of the latter divides again in the same manner, and 

 so the process of subdivision of existing stigmata goes on. 

 In this way six transverse rows of stigmata arise. These 

 may be distinguished as secondary stigmata, since they 

 arise by division from the primary. 



Gradually, by a peculiar process of growth, the long 

 axes of the secondary stigmata change their direction, and 

 instead of lying transversely they become directed antero- 

 posteriorly. This is their definitive position, and the 

 stigmata now go on rapidly dividing again, and the num- 

 ber of transverse rows of stigmata is in this way doubled, 

 trebled, quadrupled, etc., and we thus arrive at the adult 



