274 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, 



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, pc, pericardial cavity. //./, pleural cavity. Ig, lung. /, alimentary 

 tract, ao, dorsal aorta, ch, notochord. rp, rib. st, 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 permita profitable discussion of the subject here. 



