DEVELOPMENTAL EVOLUTION 235 



is most elaborately provided for, anatomically and 

 physiologically, even to the physical extent that it is 

 universally surrounded and interpenetrated by a fluid 

 medium (Figs. 98, 99), in which it floats, free from the 

 exigencies of an ever-changing environment, and sustained, 

 by a series of anatomical structures of a marvellously 

 perfect order, subservient to its every requirement, and 



FIG. 97. BRAIN AND SPINAL CORD EXPOSED FROM BEHIND IN A FCETUS 

 OF THREE MONTHS. (From Kolliker.) 



fa, the hemispheres ; ;, the mesencephalic vesicle or corpora quadrigemina, c, the 

 cerebellum ; below this are the medulla oblongata, mo, and fourth ventricle, with 

 remains of the membrana obturatoria. The spinal cord, s, extends to the lower 

 end of the sacral canal, and presents the brachial and crural enlargements. 



amenable, to some degree, to the control of the will, 

 when it becomes perfectly evolved. 



The organs of sense are placed in the most favourable 

 positions to secure information from the external world, 

 and supplied with receptive organs, capable of receiving 

 information from all points, and from all forms of material, 

 and energy, and conveying them, in the most specialised 

 forms, through appropriate neural channels, to the sen- 

 sorium within, when it, in turn, becomes fully evolved. 



At this early period, moreover, are provided the " ways 

 and means," by which the vis medicatrix nature is enabled 

 to maintain a hygienic condition of the systemic nervature, 



