THE ANCESTRY OF THE VERTEBRATES 635 



Another objection raised against the annelid hypothesis has been 

 the impossibility of comparing the mouth of the annelid with that of 

 the vertebrate. The acceptance of the annelid hypothesis makes it 

 necessary to assume that the transformation of the annelid into the 

 vertebrate has involved the loss of the annelid mouth and the formation 

 of a new mouth ventral to the nervous system. 



Ley dig, in 1864, was the first to suggest that the esophagus formerly 

 passed through the vertebrate brain in the region of the fourth ventricle, 

 between the crura cerebri. The late appearance of the mouth in the 

 vertebrate embryo has been interpreted as supporting this assumption. 

 Dohrn (1875) pointed out the fact that the embryo of a vertebrate is 

 almost completely formed, that all other organ systems are developed, 

 and that circulation has begun before the mouth opens, while in inverte- 

 brates the mouth is one of the first organs formed. To some morpholo- 

 gists, however, there appears to be insurmountable difficulty in assuming 

 that the gut of vertebrates once pierced the brain, either in the region 

 of the infundibulum or between the crura cerebelli. Of such a relation, 

 ontogenesis affords not the slightest evidence. 



To meet this difficulty. Beard 1888 assumed that when the annelid 

 became a vertebrate the supra-esophageal ganglion disappeared, so that 

 the nerve cord came wholly from the ventral cord of annelids. The 

 assumption that the esophagus of vertebrates once pierced the nerve 

 cord was thus rendered unnecessary. 



Speaking of the annelid hypothesis, Minot (1897) says, "As regards 

 the head, however, even theories have hitherto failed us, and no hypothesis 

 as to the evolution of the vertebrate head from the annelidan type has 

 been brought forward, which could not be shown to encounter insuperable 

 objections, or at least which appeared insuperable to those who were 

 opposed to the annelid theory." 



As his contribution of the solution of this problem, Minot suggested 

 that, in the evolution of a vertebrate, the paired lobes of the supra- 

 esophageal ganglion of the annelid failed to unite in the median plane, 

 and were converted into the lateral eyes of vertebrates. "We are thus 

 led to the supposition that eyes and optic nerves represent in vertebrates 

 the supraesophageal ganglia and the esophageal commissures of the 

 articulates." Thus, according to Minot, the brain of vertebrates arose 

 from the subesophageal and adjacent ventral ganglia of the annelid. 

 Its enlargement would cause the brain to expand forward and carry 

 the mouth to a new position on the hemal or heart side, and at the same 

 time prevent the eyes and preoral ganglia from meeting in the median 

 line and force the visual organs into lateral positions. "We thus reach a 

 natural and simple explanation of the difference between the annelid and 

 vertebrate head." Minot stresses the similarity of the ectodermal invagi- 



