230 THE ASCIDIANS. 



formed stigmata a minute portion becomes gradually con- 

 stricted or nipped off. Thus between and cut off from the 

 two original stigmata, there come to lie two intermediate 

 stigmata of much smaller size. (Cf. Fig. 107.) 



In this way, then, in Ciona, we arrive at the stage with 

 four branchial stigmata on each side of the pharynx. For 

 convenience we shall refer to these by the Roman nu- 

 merals, I., II., III., and IV. It is a remarkable fact that 

 II. and III. do not arise by new perforations, but are cut 

 off from I. and IV. respectively. 



On account of the close relations which the two first- 

 formed stigmata, I. and IV., bear to one another during 

 the production of the intermediate stigmata, their ventral 

 extremities coming into contact and apparently some- 

 times fusing together so that II. and III. might almost 

 be described as a joint production of I. and IV. rather than 

 as entirely independent offshoots, one is forced to the 

 conclusion that the two first-formed stigmata themselves, 

 though they actually appear simultaneously as separate 

 perforations, in reality represent the two halves of a 

 single primitive gill-slit divided into two by a tongue- 

 bar. If, moreover, we examine the exact origin of these 

 two stigmata (I. and IV.) by means of transverse and 

 horizontal sections, we may become convinced that such 

 is indeed the case ; namely, that they represent the two 

 halves of a primitive gill-slit which, on account of the 

 precocious formation of the tongue-bar between them, 

 become perforated separately. 



For the formation of any two or more consecutive gill- 

 slits, we usually expect to find separate endodermic pockets 

 or pouches of greater or less depth growing out towards 

 the ectoderm. (Cf. Figs. 72 and 92.) 



We ought to find something analogous to this in Ciona 



