THE EVOLUTION THEORY 119 



uniformity of structure (fig. 32). It is, however, not so great 

 that an embryologist would be unable, as was formerly thought, 

 to differentiate between embryos, for there are always more 

 or less prominent distinctions present. But on examining 

 the different organs accurately we cannot fail to notice the 

 uniformity. 



All vertebrates, for instance, form the central nervous system 

 as a dorsal gutter-like depression of the ectoderm, extending 

 from the anterior to the posterior end of the young embryo. 

 In the course of the development this gutter becomes deeper and 

 deeper, finally separating completely from the ectoderm and 

 forming the neural canal or spinal cord. In the lowest verte- 

 brate, the archetypal fish Branchiostoma the nervous system 

 retains this form permanently. Moreover, the lumen of the 

 neural canal is here uniform in thickness, except at the anterior 

 end where we can perceive in a slight vesicular enlargement the 

 first stage of the brain. 



In the other vertebrates the nervous system does not stand 

 still on this primitive stage, but makes further progress in the 

 formation of the anterior part of the brain. The neural canal 

 continues to distend, the enlargement forming three separate 

 parts, the primary cerebral vesicles. By a further division we 

 obtain the five parts of the brain, which are to-day found in all 

 vertebrates from the fishes upwards. That the brain of the 

 different classes is, nevertheless, widely different in appearance is 

 explained by the different size and formation of the five cerebral 

 vesicles. Together with the growth of the mental faculties we 

 observe in the cerebrum and cerebellum a growth as well as a 

 differentiation ; both gradually assume in birds, but especially in 

 mammals and man, a size far exceeding all other parts of the 

 brain. 



The common origin may further be perceived in the develop- 

 ment of the vascular system and respiratory organs. As it is 

 one of the principal functions of the blood to distribute the 

 oxygen received from the respiratory organs throughout the 

 whole body and to remove the carbon dioxide resulting from 

 the vital processes these two systems are strictly correlated. 

 If, therefore, on an animal leaving the water, lung-breathing 

 replaces breathing through gills the circulatory system must 



