58 



Growth in length of the Vertebrate Embryo 



either in the Chordate phylum or in the Echinoderm phylum. 

 In both these phyla openings from the gut or its pouches are 

 common as they are in the Coelenterate. In the coelenterate we 

 find numerous apertures in the tentacles and in the body wall, as 

 for instance the cinclides of Anemones. In Chordates apertures of 

 this nature are numerous, e.g. the gill clefts of any class ; or the 

 intestinal pores of certain Hemichordata. 



In the Echinoderm also the mouth is the new opening. The 

 hydropore is also a new opening into an archenteric diverticulum. 

 It may be objected that gill clefts are essentially exhalent and not 

 inhalent apertures. No one, however, would deny the homology 



II 



Fig. 32. Diagram to show the closure of the blastopore, and to show the part 

 that remains open as anus, an. I, Urodela; II, Anura. 



of the spiracles of the Dog-fish with the branchial clefts. It is 

 almost certain that they must at one time have been exhalent 

 apertures. Now they are inhalent. There is no reason why the 

 present mouth should not have arisen from either an inhalent or 

 exhalent pore, provided it occurred in a suitable place. 



In both the Chordata and Echinodermata we find strong 

 embryological evidence for the assumption that the blastopore 

 became anus only and that the present mouth is a new pore. In 

 every case described the anus is always either derived directly 

 from the blastopore or else it opens at the spot where the blastopore 

 has closed. Thus the Chordata and Echinodermata agree in their 

 main axial relation to the plane of the blastopore, for, in each case 

 it is at right angles to this plane. They also agree in various other 



