712 



Teratogenesis 



these abnormalities after similar treatment, 

 but also showed a much greater incidence of 

 spontaneous duplicate embryos. Holtfreter 

 ('45) has demonstrated that a difference in 

 neuralization response between the ectoderm 

 of Amblj'stoma punctatum and Triturus toro- 

 sus is the result of an increased susceptibility 

 of the former to hypertonic salt solutions. This 

 susceptibility is in turn related to differences 



is injected into the yolk sacs of unincubated 

 eggs than do embryos of other breeds of 

 chickens. Moreover, it is possible (Landauer 

 and Bliss, '46) to select a "high" and a "low" 

 susceptibility line from amongst the White 

 Leghorns themselves (progeny from hens 

 giving high and low incidences of rumpless- 

 ness were saved and inbred). After continued 

 selection the susceptibility of the high line 



40 50 60 10 80 90 100 110 120 130 140 150 160 HOUTS 



Time oi ZniQction of Tjisulin 



Fig. 247. Effect of time of injection of 2 units of insulin on the incidence of rumplessness and 

 micromelia. All injections into yolk-sac of embiyos of ages indicated. The continued small incidence 

 of rumplessness beyond 72 hours is the same as the sporadic occurrence in controls. (Data from Landauer 

 and Bliss, '46; Landauer, '47a.) 



in the resistance of the cell membrane of 

 the outer ectodermal cells. 



The above examples refer to differences 

 between genera. Similar differences may be 

 found between different strains within a 

 species. Warkany et al. ('42) demonstrated a 

 strain difference between Sprague-Dawley 

 and Wistar rats in regard to the production 

 of skeletal abnormalities as a result of dietary 

 deficiencies (later shown to be riboflavin). 

 The former strain had a higher incidence 

 of abnormalities. However, this was shown 

 to be due to its greater resistance to the 

 deficiency, since the Wistar rats were so 

 affected by this treatment that they failed 

 to produce any young. An even more strik- 

 ing demonstration of genetically conditioned 

 response to experimental conditions has been 

 provided by Landauer ('48b). White Leg- 

 horn embryos yield a considerably higher 

 percentage of rumpless embryos after insulin 



increased to such an extent that, with 2 

 units after 4 hours of incubation, the inci- 

 dence of rumplessness was 51.9 per cent 

 while in the low line it was 18.2 per cent 

 (Landauer, unpublished). This increased 

 susceptibility to injection of insulin at early 

 stages is not retained at later stages. When 

 insulin is administered at 5 days there is a 

 high incidence of micromelia. This incidence 

 is no greater in the high line (i.e., high in 

 regard to production of rumplessness) than 

 in the low line. 



STAGE OF TREATMENT 



It has been recognized for some time that 

 the stage during which an embryo is sub- 

 jected to a particular type of treatment is 

 of utmost importance in determining the 

 type of abnormality which the treatment 

 will cause. In general the younger an em- 



