Scharff — On Dohrn's Theories on the Origin of Vertebrates. 37 



embryos of Accipenser. As a result of these studies, he feels 

 inclined to regard the branchiostegal membrane of teleosteans as 

 the homologue of the opercular gill of ganoids. 



XII. — Criticism of Recent Memoirs. 1 



In the most recent of Dohrn's studies a preliminary 

 notice on the morphology of tunicates by Van Beneden and Julin 

 is discussed by the author. According to their researches, the 

 whole body of a fully-grown ascidian is only homologous with the 

 two most anterior segments of Amphioxus. The gill-clefts of 

 Amphioxus and tunicates are therefore not homologous either with 

 Petromyzon, or with those of fishes. The heart of an ascidian, they 

 say, and the heart of vertebrates are completely different forma- 

 tions, for one is endodermal, while the other is a mesodermal for- 

 mation. The same authors not only attack Dohrn's theories in 

 general, but throw doubt on some of the facts ascertained by him. 



It will be remembered that Dohrn laid particular stress on the 

 homology of the thyroid in fishes, and the hypobranchial groove 

 in tunicates and Amphioxus. His views were based especially on 

 the existence of a pseudobranchial groove, which he compared with 

 the ciliated oesophageal groove of tunicates. 



Yan Beneden and Julin doubt the identity of the ciliated 

 apparatus in tunicates with that in Ammocoetes on account of the 

 apparent difference in their constitution. This difference, however, 

 according to Dohrn, only lies in a misconception on the part of 

 the author's of Schneider's description of the pseudobranchial 

 groove in Ammocoetes. 



Along with a full Paper on this subject, Van Beneden and 

 Julin also propose to publish shortly their researches on the inner- 

 vation of the branchial apparatus of elasmobranchs and cyclostomes. 

 According to Dohrn's opinion, which, by the way, differs from 

 Van Beneden and Julin's, there is not the slightest doubt, as 

 proved by the position of the nerves, that the gill-pouch which 

 forms the pseudobranchial groove in Ammocoetes is homologous 

 with the spiracle of elasmobranchs. It is further proved by the 



1 Ibid., vol. vii. 1887, pp. 301-337. 



