No. 2.] DEVELOPMENT OF PETROMVZON'. 279 



gill-cleft. Not having seen Julin's figures as yet, I do not know 

 upon what evidence his statement rests, but I think it must be 

 a mistake. In the adult the facial has no such distribution. 

 Fiirbringer's (14) account of this nerve m Petromyzon is as 

 follows : — 



" Wenig kraftiger, rein sensibler nerv. Tritt mit dem 

 Acusticus durch eine Oeffung in die Gehorkapsel, durchbohrt 

 den vorderen Umfang derselben schrag nach unten und vorn 

 um dicht an der Aussenflache derselben den 



" i) Ramus recurrens abzugeben, der sich in nahezu hori- 

 zontalem Verlauf nachriickwarts um die Aussenflache der Gehor- 

 kapsel schlagt um mit dem Ramus lateralis des Vagus, nahe 

 dessen Abgang von letzterem zu verschmelzen. Die Fortset- 

 zung des Stammes verlauft weiter nach vorn und lateralwarts, 

 zwischen Auge und subocularem Bogen, sich mit dem Haut- 

 aste des Ram. ext. Trigem. unter spitzem VVinkel kreuzend 

 iiber ihn tretend. Kurz vor der Kreuzungsstelle giebt er den 



"2) Ramus posterior ab, welcher nach hinten, aussen und 

 unten verlaufend zwischen der dorsalen und ventralen Portion des 

 Seitenrumpfmuskels unter die Haut dringt in dieser zwischen 

 Auge und i. Kiemenloch veriistelnd." 



I have satisfied myself of the accuracy of these statements, 

 and also that in the adult the first permanent cleft (second of 

 the primary series) is supplied by the ninth, or glosso-pha- 

 ryngeal nerve, which is in harmony with the account given by 

 Ahlborn (2). Sagittal sections of late embryos and very young 

 larva; clearly show the branches of the facial forking over and 

 supplying both sides of the first (primary) cleft. Shipley also 

 states that this nerve is distributed to the circumoral ciliated 

 groove, in agreement with Dohrn's description. It would be 

 exceedingly remarkable if the older larvae should turn out to 

 differ so radically both from the embryo and the adult with 

 regard to the distribution of the seventh pair of nerves, as is 

 indicated in Julin's statements. But even admitting the cor- 

 rectness of his observations, there can be no possible doubt 

 that the embryo of Pctroviyson possesses eight pairs of bran- 

 chial clefts. Huxley's account of this (18), which first directed 

 my attention to the subject, is incorrect, as at the stage which 

 he describes and figures the first pair of pouches has long 

 since been converted into the ciliated groove, but in the embryo 



