EMBRYONIC DEVELOPMENT 397 



spina bifida occurred in 21.7 per cent of the individuals. Hertwig (52), 

 and also Baldwin (15, 16, 17), have reported the production of large 

 numbers of animals with this defect. Recently abnormal gastrulation 

 in radiated amphibian eggs has been studied by Simon (87), by Olivieri 

 (69), and by Pasquini and Meldolesi (76, 77). Pasquini and Meldolesi 

 have found early stages of gastrulation to be more sensitive than are later 

 stages. 



MODIFICATIONS IN LATER STAGES OF DEVELOPMENT 



Except for the observations of Perthes (79), Tur (90, 91), and a few 

 others, most of the detailed studies of developmental- abnormalities which 

 occur after radiation have been made on amphibian larvae and on chick 

 embryos. With regard to amphibians the most complete accounts will 

 be found in the papers by Schaper (85), Levy (63), Hertwig (52), and 

 Bardeen (18, 19, 20). A survey of this work reveals that the external 

 defects which are of most frequent occurrence are abnormally shaped 

 heads, distended abdomens, and deformed or dorsally flexed tails. Exam- 

 ination of the internal organs shows that the nervous system, the eyes, 

 and the vascular system appear to be most susceptible to injury. Any 

 or all of these abnormalities may occur without reference to the partic- 

 ular time at which radiation w^as made. Bardeen has shown that many 

 of the abnormalities occur in experiments in which either the spermatozoa 

 or the ova were radiated before fertiUzation. On the other hand, 

 abnormalities are frequent also in cases where radiation was administered 

 in relatively late stages of development. 



Radiation of chick embryos in ovo results in extreme developmental 

 disorders and often in the death of the embryo. Gaskell (38), Colwell, 

 Gladstone, and Wakeley (30, 31), Ancel and Vintemberger (2), Regaud, 

 Lacassagne, and Jovin (80), Strangeways and Fell (88), Heim (44), and 

 Butler (28) all describe in considerable detail the changes which occur as 

 a result of various dosages of radiation. In chick embryos, as in amphib- 

 ian larvae, vascular disorders are common. These vascular disorders are 

 undoubtedly associated with the fact that the vascular system is one of the 

 first to be established and also is one of the most rapidly developing sys- 

 tems of the body. Moreover, the extra-embryonic circulation of the 

 chick embryo is so extensive that small initial developmental arrests 

 probably result in later and more extensive abnormaUties in the embryonic 

 and extra-embryonic circulatory pattern. One of the most obvious and 

 also one of the most common occurrences is to find many areas of extrav- 

 asated blood, both within the body and also in the extra-embryonic 

 area of radiated chicks. Furthermore, in addition to vascular disorders, 

 abnormal development of the nervous system, particularly the brain, is of 

 frequent occurrence in radiated chick embryos. These nervous abnor- 

 malities conform to no particular pattern. They appear to be the result 



