IO2 



ORGANIC EVOLUTION 



higher animals when adult have two apertures into the diges- 

 tive tract, the mouth and the anal aperture, but in their devel- 

 opment they pass through a stage when like the jellyfishes they 

 have only the one opening (Figs. 23 and 24). This single em- 

 bryonic aperture is called the blastopore and is a reminiscence 

 of the jellyfish mouth. In certain of the lower vertebrates, 

 the frog for example, we find the blastopore present in the 

 embryo and well formed and functional (Plate 42, A and B\ 



FIG. 23. Gastrula of a coral polyp (Monaxenia darwinii) . [After HAECKEL.] 

 a. A surface view. b. A longitudinal section. 



Later it closes and disappears. In the higher vertebrates, 

 on the other hand, the blastopore does not become functional 

 at any time during the embryonic life (Plate 42, C). It is 

 a nascent organ. It begins to appear, but never reaches 

 normal development, and later disappears without ever hav- 

 ing come to its typical condition. Its presence is of no use 

 to its possessor, so far as we can see, but the fact that it is 

 there in a rudimentary condition agrees with our principle 

 that the development of the individual tends to recapitulate 

 the evolution of the race. The ancestors of the vertebrates, 



