536 ON INCREASE IN SIZE [pt. iii 



the short ones at the violet end of the spectrum. It would be very- 

 desirable to go into the whole subject anew in the light of modern 

 technique and modern conceptions. 



2- 1 8. The Effect of X-rays and Electricity on Embryonic 

 Growth 



Other forms of radiant energy have also been investigated, but the 

 position is very complicated and unsatisfactory. Oilman & Baetjer 

 in 1904, judging from morphological and cytological evidence, con- 

 cluded that in Amblystoma, X-rays at first accelerated development 

 and then retarded it so that eventually the exposed embryos were 

 smaller than the controls. 



For the silkworm Hastings, Beckton & Wedd have asserted that 

 X-rays accelerate the developmental time, and (Hastings) that the 

 secondary radiation excited by irradiating copper retards the de- 

 velopment of the silkworm. Ancel & Vintemberger in an elaborate 

 and long research, decided that in the case of the chick and the 

 frog, X-radiation had neither an accelerating nor a retarding in- 

 fluence. Nevertheless Colwell, Gladstone & Wakeley believed that 

 they had evidence of retardation by X-rays in the chick, as the 

 following table shows: 



Table 76. 



Their second paper, though mainly morphological, confirmed the 

 earlier one. Richards obtained acceleration working with the egg 

 of the mollusc Planorbis, and the opisthobranch Haminea virescens. 

 Packard working with Arbacia, Lazarus-Barlow & Beckton with 

 Ascaris, Bohn with Strongylocentrotus eggs, found that short exposures 

 to radium accelerated development and long ones retarded it. 

 Lazarus-Barlow & Beckton found that /S and 7 radiations alone had 



