Scharff — On Dohrn's Theories on the Origin of Vertebrates. 35 



dorsum. As Dohrn attempted to prove in previous chapters, no 

 trace of myotomes were to be found in the head, and the apparently- 

 dorsal optic muscles were branchial muscles. In this case also 

 it is very natural to look upon the cartilaginous parts which the 

 muscles are inserted on as transformed visceral arches, whichever 

 portions of the primordial cranium they may belong to. As to the 

 cranial nerves, Dohrn has not yet come to a definite conclusion. 

 His opinion, at present, is that they have lost those branches which 

 innervated the myotomes and their derivatives. In consequence, 

 however, of the extraordinary enlargement and complication of the 

 ventral region of the head, the cranial nerves have gained all the 

 more. They have become greatly complicated in their course, due 

 to the manifold changes of position among the branchial arches. 

 The spinal nerves, as a whole, have been less modified than the 

 cranial nerves. In the caudal region, however, they have been 

 deprived of their entire visceral parts, and, consequently, they are 

 here least complicated. 



The above hypothesis will enable us, continues Dohrn, to draw 

 other conclusions, viz. that the dorsal roots of the cranial nerves 

 have become lost along with the corresponding muscles, and that 

 the attempts of Yan Wijhe and others to diagnose dorsal branches 

 are founded in mistaken views. 



XI. — The Spiracle and the Pseudobranch} 



After having reviewed passages from a number of authorities 

 on the spiracle, Dohrn explains the course of blood-vessels of the 

 same organ in elasmobranchs. This has, however, been considered 

 before in the seventh study. 



It is a most peculiar fact, says Dohrn, that the aerated blood 

 from the hyoid gill finds its way directly, by means of the cross 

 commissure, into the spiracular artery. It then circulates once 

 more through the lamellae of the spiracular gill, before entering 

 the cephalic circle. It is evident that this condition cannot have 

 obtained originally, and it must be presumed that the spiracle once 

 upon a time received blood directly from the conus arteriosus. In 



1 Ibid., vol. vii. 1886, pp. 128-176. 



