6 04 



ON INCREASE IN COMPLEXITY 



[PT. Ill 



wards from the animal pole. Eggs in an early gastrula stage always 

 disintegrated first at the dorsal Up region and shortly afterwards 

 in the same meridian about 120 to 130° above the blastopore. 

 From the upper point at the animal pole the wave spreads down- 

 wards, and meets the disintegrated area of the dorsal Up, which has 

 spread apically and now includes the lateral lips, after which all 

 the pigmented cells become 

 gradually involved, though the 

 cells at the vegetal pole retain 

 their structure with their yolk 

 long after the rest of the egg has 

 died. Lateron, when elongation 

 has begun, the dorsal lip now 

 takes up the posterior position, 

 and the double gradient still per- 

 sists, disintegration beginning 

 from both ends, from the apical 

 point at the anterior end and 

 from the dorsal lip at the pos- 

 terior end. This description 

 applies to most of the later 



Fig. 103. 



stages, including the time of opening and closing of the neural folds, the 

 appearance of ventral suckers, etc. Eventually, with the differentia- 

 tion of various organs, local susceptibility differences begin to appear, 

 and the tail bud, the optic vesicles, the nasal pits, and other rapidly 

 growing regions show much susceptibility. (See Fig. 103.) 



On the basis of these fundamental results, Bellamy was able to 

 make and verify teratological predictions, and to control experi- 

 mentally or modify development, e.g. the gastrular angle and the 

 cleavage ratio (the ratio between the sizes of animal and vegetal 

 pole cells). He was criticised by Cannon, whose main objection was 

 that the effects of the toxic agents were not uniform at the different 

 stages, but that the individual differences between eggs were so 

 large as to invalidate Bellamy's conclusions. When Cannon did 

 succeed in getting a lot of eggs to behave in the same way at the 

 same time, he found results quite at variance with Bellamy's, e.g. 

 the ventral, not the dorsal, region of neural tube stages was the 

 more highly susceptible. These criticisms were replied to in detail 

 by Bellamy & Child, who successfully rebutted them, and whose 



