of Edinburgh , Session 1884 - 85 . 
9 
was related to this theory very much as Lamarck’s notions on 
evolution to the “ Origin of Species.” 
The correspondence between a Chaetopod turned over, and say 
a fish, is very complete. On the upper side (ventral in Chaetopod’s 
natural position) is the double nerve-cord ; so also in the fish. In 
the axis is the intestine. Above the intestine in the inverted 
Chaetopod is a blood-vessel, in which the blood passes from the head 
towards the tail (subintestinal vessel in natural position). So also 
in the fish — the dorsal aorta. In the inverted Chaetopod, below 
the intestine, is a blood-vessel in which the blood passes from the 
tail to the head (dorsal blood-vessel in the natural position). So 
also in the fish — (subintestinal vein, heart, ventral aorta). 
Moreover, in the embryo of Chaetopod, Insect or Crustacean, the 
blastoderm is formed on the ventral surface, along which the primi- 
tive blastopore extends, though in n^ost cases its actual opening is 
narrowed to a small posterior aperture, which becomes the anus. In 
the Vertebrate embryo the blastoderm is dorsal, and, as Sedgwick* 
has shown in a recent paper, and as Miss Johnson f has confirmed, 
there is evidence in the Vertebrate embryo of a fusion of the layers 
at an early stage along the median line of the blastoderm, indicating 
a primitive elongated blastopore. 
Sedgwick’s view of the morphology of Vertebrates differs from 
Dohrn’s in one most important point. The former considers the 
actual mouth and anus of Vertebrates to be identical with the 
primitive mouth and anus, and supposes that a portion of the 
nervous system, representing the supraoesophageal ganglia of Chaeto- 
pods, &c., has disappeared. On Dohrn’s view, the primitive anus 
was terminal, and is represented (when the actual blastopore is 
enclosed by the medullary groove) by the neurenteric canal. Where 
was the primitive mouth if the actual mouth is a secondary struc- 
ture ? Dol;rnJ pointed out clearly in 1875 that a primitive 
mouth must have existed, surrounded, as in modern Chaetopods, by 
nerve-cord. He first supposed that the hypophysis cerebri, or 
pituitary body, then believed to be developed from the intestine, 
was the rudiment he was seeking ; but in the publication I have 
referred to he definitely abandoned this view, and took up the theory 
* Q. J. M., January 3, 1884. t Ibid,, October 1884. 
+ Ur sprung der Wirbelthiere , Leipzig, 1875. 
