EMBRYOLOGY 145 



the highest Metazoa might have been evolved, includ- 

 ing the enormous ascent from sponge to man. In 

 this it would be necessary to show the evolution of 

 the vertebrate from the invertebrate animal. 



At this point we would naturally look to embryol- 

 ogy to bridge the chasm. Dr. Romanes, however, 

 who more than almost any other writer has called to 

 his aid embryology in trying to establish the truth of 

 the theory of evolution, does not attempt to use it in 

 trying to show the evolution of the vertebrate from 

 the invertebrate. He claims that the lowest verte- 

 brate, the Amphioxus, is related in structure to 

 Balanoglossus, a living species of worms, and that 

 the two probably had a common ancestor. 



It is well known, however, that there are radical 

 differences between the embryos of vertebrates and 

 invertebrates. Worms and other articulates in 

 embryo lie doubled backwards around the yolk, while 

 all vertebrates are doubled in the opposite direction. 



According to the theory that the embryonic condi- 

 tion is a recapitulation of the stages of organic evo- 

 lution, this -fundamental fact of invertebrate embry- 

 ology ought to have been preserved by the vertebrate, 

 and it ought, at least, to pass through a stage of de- 

 velopment bent backwards around the yolk. Evolu- 

 tion gives no account of this reversal of position by 

 the vertebrates. 



Again, among the vertebrates are found three mod- 

 ifications of development. Fishes and Amphibians 

 have no amnion nor allantois; Reptiles and birds 

 have both, while the Mammals alone have a placenta. 

 Evolution gives no account of these fundamental dif- 

 ferences. 



It is true that the embryology of the vertebrates 

 shows that at an early stage of development the em- 

 bryos of all vertebrates closely resemble each other in 

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