2 74 Vertebrate Embryology 



process of development may be fairly accurately de- 

 fined as equivalent to histogenesis. As to the factors 

 which cause differentiation, we have no satisfactory 

 knowledge. We can, at present, only note the changes, 



S/3. c 



Fig. 84. — Section through an advanced embryo of a rabbit, 

 to show how the pericardial cavity becomes surrounded by 

 THE PLEURAL CAVITIES. (After Foster and Balfour.) 



ht, heart, /c, pericardial cavity, //./, pleural cavity, /^, lung, rt/, alimentary 

 tract. ^(7, dorsal aorta, c^, notochord. r^^rih. j^, sternum, j/.r, spinal cord. 



when they acquire such magnitude as to become micro- 

 scopically visible. As to the physiological conditions 

 which cause these changes we have almost no concep- 

 tions. It is probable that the nucleus has a leading role 

 to play, but our knowledge of this role is too litte ad- 

 vanced to permit a profitable discussion of the subject here. 



