RELATIVE STABILITY OF LIVING MATTER 



341 



doubt in nature similar temporary effects are caused by internal 

 stress or pressure of the parts of the egg itself. 



After noting other causes of irregularity, Wilson fittingly 

 observes : 



All these considerations drive us to the view that the simpler mechanical 

 factors, such as pressure, form, and the like, are subordinate to far more 

 subtle and complex operations involved in the general development of the 

 organism. . . . We cannot comprehend the forms of cleavage without refer- 

 ence to the end results}- 



Promorphology of the ovum. Is there something in the origi- 

 nal shape or character of the egg that corresponds to the fin- 

 ished individual ? Is there a polarity of the egg that is in any 

 way related to the order of cleavage and the axis of the body ? 



Speaking of the eggs of insects, Wilson says : 2 



In a large number of cases the egg is elongated and bilaterally sym- 

 metrical, and, according to Blochmann and Wheeler, may even show a 

 bilateral distribution of the yolk corresponding with the bilaterality of 

 the ovum. 



Hallez is here quoted as asserting, after a study of the cock- 

 roach, water beetle, and the locust, that " the egg cell possesses 

 the same orientation as the maternal organism that produces it ; 

 it has a cephalic pole and a caudal pole ; a right side and a left ; 

 a dorsal aspect and a ventral ; and these different aspects of the 

 egg coincide with the corresponding aspects of the embryo." 

 Wheeler, after studying some thirty different species of insects, 

 reached the same result, and concluded that even when the egg 

 approaches the spherical form- the symmetry still exists, though 

 obscured. 3 



In species other than insects the egg often has a bilateral 

 symmetry of its own, "sometimes so clearly marked that the 

 exact position of the embryo may be predicted in the unferti- 

 lized egg." 3 



Polarity of the ovum. It is now well known that in the seg- 

 menting eggs of the frog the first two cleavage planes are vertical, 

 the first corresponding to the median plane of the body and set- 

 ting off right and left halves (which develop into corresponding 



i Wilson, The Cell, pp. 376-377. 2 Ibid. pp. 383-384- 3 Ibid - P; 384- 



