ORGANIC EVOLUTION 



201 



of the central cavity of the medulla, but thinly covered 



upon the dorsal side. 



The elongate brain of the sala- 

 mander, with its parts outspread 

 almost like a diagram, is very 

 simple in comparison with that of 

 the higher vertebrates (fig. 128). 

 In birds the cerebellum and the optic 

 lobes are relatively larger and in 

 mammals, and especially in man, 

 the cerebral hemispheres attain their 

 maximum development. 



Thus a simple tube of undifferen- 

 tiated nerve cells becomes moulded 



a brain - B localized OUt- 



FIG. 128. Brains*, of pigeon 



: growths of its walls all the principal 

 external features of its form are 

 p. optic lobe. wrought out. The subsequent de- 



velopment of fibres from all these masses of cells is a 

 matter far too intricate for us to attempt to follow here. 

 A few of the more salient features of the ultimate distribu- 

 tion of these fibres will be considered in chapter VII. 



The development of the primary circulation. In the 



midst of the mesoderm, tube-like clefts appear, which, 

 extending and becoming confluent, develop into the blood 

 vessels. The most important of these appears as a cleft 

 of sigmoid curvature in the region of the throat, and by 

 the processes diagrammatically represented in figure 129, 

 it becomes enlarged, strongly flexed, and divided into com- 

 partments, it develops muscular walls, and becomes the 

 heart. It becomes two chambered by the differentiation 

 of an auricle and a ventricle, the former being carried to 

 the front of the latter by the flexion undergone during 



