ANATOMY AND DEVELOPMENT. 



231 



if the two first-formed stigmata had the value of indepen- 

 dent gill-slits. 



Instead, however, of anything approaching to two endo- 

 dermic outgrowths, we find at the base of the atrial invo- 

 lution a single endodermic ingrowth making its appearance 

 (Fig. 108). 



The angles made by this ingrowth with the neighbour- 

 ing wall of the branchial sac remain in contact with the 

 floor of the atrium, then fuse with it, and finally become 





af 



wwM'mm 



en 



Fig. 108. — Dias^rams illustrating the mode of origin of the two first-formed 

 branchial stigmata in Ciona. (After WiLLEY.) 



at. Atrial involution, ec. Ectoderm. en. Endoderm. g.s. Stigmata. t.b. 

 Tongue-bar. 



perforated (Fig. 108). This is the way in which the stig- 

 mata, I. and IV., arise, and it is difficult, if not impossible, 

 to interpret the above-mentioned endodermic ingrowth 

 otherwise than as 2l precocious tongue-bar. 



Even in Amphioxus it was seen how the tongue-bars 

 of the secondary slits arose relatively much earlier than 

 those of the primary slits. If they arose still a trifle 

 earlier, we should have the two halves of each slit becom- 

 ing separately perforated, just as it happens in Ciona. 

 In a species of Balanoglossus an analogous precocious 



