THE ANCESTRY OF THE VERTEBRATES 637 



(1884) accepting this hypothesis assumed that the neural tube originated 

 in animals as a dorsal groove which secondarily became converted into a 

 canal opening into the archenteron. Van Wijhe (1884) reached inde- 

 pendently the same conclusion, but assumed that the neural tube may 

 have functioned as an ingress for food and as an outlet for wastes. T. H. 

 Morgan (1890) has supported this view. 



Delsman finds many points of agreement between the medullary tube 

 of chordate embryos and the stomodeum of Protostomians. In the 

 development of both groups, the border of the blastopore, originally 

 wide and large, contracts to a very narrow opening, the definitive blasto- 

 pore. In both annelids and chordates, there is formed, in connexion with 

 the blastopore, a tube of ectodermal origin, which at one end opens to the 

 exterior, and at the other end leads to the archenteron. In annelids this 

 ectodermal tube is the stomodeum, the outer opening being the mouth, 

 the inner the "cardiac pore" which is the opening of the stomodeum into 

 the annelid stomach. In chordates the ectodermal tube is the neural 

 tube with its neuropore and neurenteric canal. The fundamental simi- 

 larity of the relations in the two groups is obscured by the fact that the 

 "cardiopore" of the annelid lies at the anterior end of the enteron, while 

 the neurenteric canal of chordates lies at the posterior end. 



As additional proof of the comparability of chordates and annelids, 

 Delsman calls attention to the fact that the stomodeum of annelids, 

 like the neural tube of a young amphioxus, is ciliated. Morgan ('90) has 

 found that the neural tube of frogs is ciliated. Delsman sees significance 

 in the fact that "while all other developmental processes of segmented 

 animals proceed from in front backwards, the formation of the neural 

 tube in lower chordates has its starting point at the blastopore and 

 proceeds in a forward direction." 



If we assume that in chordates the stomodeum of invertebrates has 

 become elongated so as to carry the "cardiac pore" backwards to the 

 hinder-end of the body, the agreement between stomodeum and the 

 embryonic neural tube becomes almost complete. Delsman asserts 

 that in ontogenesis of vertebrates the backward shifting of the blastopore 

 is quite evident. Thus the conditions arise which necessitate the forma- 

 tion of a new mouth. That the definitive vertebrate mouth is a new 

 mouth is almost universally agreed. 



"The primary mouth, being that of annelids," says Delsman, "is 

 represented by the neuropore of amphioxus, and this itself again is 

 phylogenetically secondary in respect to the "Urmund," the mouth of the 

 hydroid polyps, which in annelids we find again in the cardiac pore, and 

 in chordates in the neurenteric canal, both representing the former 

 blastopore. Thus in the ontogeny of vertebrates we see the three suc- 

 cessive mouths appear in the same succession as they appeared in phylo- 



