RADIATION IN PRENATAL DEVELOPMENT 863 



abnormalities at birth. It should be pointed out that the division points 

 do not represent sharp demarkations but merely separate average 

 response for entire periods. Appropriate groupings of the work of other 

 authors support, in general, these broad divisions of prenatal development 

 with respect to response to radiation. Each division will, therefore, be 

 taken up in turn. 



An attempt at complete coverage of the literature has been made. The 

 experimental material consists almost entirely of mice, rats, guinea pigs, 

 and rabbits. In the publications here reviewed, the number of investi- 

 gators, or groups of investigators, concerned with each of these is 11, 9, 8, 

 and 15, respectively. Incomplete data are available on two cats (Tous- 

 sey, 1905; Schinz, 1923) and one dog (Regaud et al., 1912). Only the 

 mouse and rat have been used in recent experiments, all work with other 

 species having been published before 1935. In order to determine broad 

 developmental divisions, similar to those described in the preceding 

 paragraph, for the species to be discussed, landmarks in development 

 (e.g., blastocyst, first mesoderm, neural plate, first somite, etc.) were 

 entered against coordinates representing days of gestation of mouse and of 

 the animal to be compared with it. It is thus possible to arrive at the 

 following approximate divisions for preimplantation period, period of 

 major organogenesis, and period of the fetus, respectively: mouse — 0-5, 

 6-13, 14 to term (usually days 19 or 20) ; rat — 0-7, 8-15, 16 to term (usu- 

 ally days 21 or 22) ; rabbit — 0-5, 6-15, 16 to term (usually days 31 or 32) ; 

 guinea pig — 0-8, 9-25, 26 to term (approximately 9 weeks; but it must 

 be remembered that birth here does not occur at a comparable stage, a 

 newborn mouse corresponding, approximately, to a 34-day guinea pig 

 fetus) . 



Whenever possible, work to be reviewed has been placed in a category 

 defined by the animal with which it deals and the time in gestation at 

 which irradiation was applied (see Tables 13-1, 2, 7). A few publica- 

 tions cover two or several categories and are, therefore, listed more than 

 once. In many cases the stage is only vaguely stated (e.g., "second half 

 of pregnancy"), in others, although not stated at all, it can be approxi- 

 mately calculated from incidental information (e.g., by counting back 

 from term). In still other cases, where a treatment interval is reported 

 (either because the irradiation was chronic or because the author is not 

 specific about the date of his acute exposure), there may be overlap into 

 an adjacent broad developmental phase. For these and other reasons, 

 Tables 13-1, 2, 7 may be considered only as organized listings of litera- 

 ture and must not be evaluated as a summary of results. It was felt 

 that this was the most profitable way of presenting the extremely hetero- 

 geneous group of investigations, many of which are not definitive by 

 themselves but may add to the picture as a whole. Not included in the 

 tables are experiments in which no clue can be obtained of the stage 



