266 J. T. CUNNINGHAM. 



1884. IV. " Die Entwicklung und Differenzirung der Kiemenbogen der 



Selachier." 

 V. " Zur Entstehung und Differenzirung der Visceralbogen bei Pe- 

 tromyzon Planeri." 

 VI, "Die paarigen und unpaaren Flossen der Selachier." 



1885. VII. " Entstehung und Differenzirung des Zungenbein und Kiefer 



Apparates der Selachier." 

 VIII. "Die Thyreoidea bei Petromyzon, Amphioxus, und den Tuni. • ) 

 eaten." 

 IX. " Die Bedeutung der Unpaaren Flosse fiir die Beurtheilung der 

 genealogischen Stellung der Tunicaten und des Amphioxus, 

 und die Reste der Beckenfiosse bei Petromyzon." 

 X. " Zur Phylogenese des Wirbelthierauges." 



Ancestral Mouth. — In the first of these studies reference is 

 made to the question of the position of the ancestral mouth, 

 which in the^ Ursprung der Wirbelthiere ' was located between 

 the crura cerebelli in the fourth ventricle. Professor Fritsch 

 and Mr. Sanders argued that this was an untenable supposition, 

 because it would be impossible to accept the consequence of 

 it, namely, that all the cerebral nerves belonged to a supra- 

 oesophageal ganglion. Dohrn acknowledges the justice of the 

 objection, and provisionally abandons the quest of the ancestral 

 mouth. He has never since resumed the inquiry. He deals 

 with investigations of the development of the actual mouth, the 

 results of which confirm his view that the aperture represents 

 a united pair of gill-clefts. In embryos of Teleosteans he found 

 that there was no stomodseum, and that the mouth arose as a 

 pair of enteric outgrowths which at first opened to the exterior, 

 one on each side, the apertures only subsequently meeting in 

 the middle ventral line. 



Hjrpophysis of Teleosteans and Petromyzon. 



The hypophysis also in Teleosteans, according to the second 

 paper of the series, does not arise from an ectodermal oral 

 invagination or stomodseum, but from a pair of endodermal 

 evaginations in front of those which form the mouth. The 

 organ therefore represents a pair of prseoral gill-clefts (i. e. it 

 is derived in the Teleosteans from the endodermal parts of 



