Teratogenesis 



713 



bryo is when svibjected to some foreign 

 environment the more profound will be the 

 alterations produced. This is frequently ex- 

 pressed by a higher incidence of very early 

 mortality or a cessation of development at 

 an early stage. Among the survivors, how- 

 ever, one may find a surprising regularity 

 in the appearance of a particular defect. 

 This has been taken to indicate that the cells 



is being most actively elaborated. This may, 

 in general, be true. However, Solberg ('38) 

 has presented material which demonstrates 

 that different structures vary considerably in 

 regard to the relationship between their 

 sensitivity to x-ray treatment and the time 

 of actual elaboration of the organ. As in- 

 dicated in Fig. 248, some organs in Fundulus 

 react to x-rays both prior and subsequent to 



EPIDERMIS-— 

 NOTOCHORDx 



G U T X- 



LENS 



PRONEPHROS 



LIVER AND PANCREAS- 



EAR AND LATERAL LINE> 



PECTORAL FIN 



CARTILAGE 



OPERCULUM AND JAWS- 

 AIR BLADDER 



GERM CELLS— 



MYOTOMES 



SPINAL CORD-" 



MID- AND HINDxBRAIN- 



FORE BRAIN « 



EYES AND NARESx 



AGE IN HOURS AT RADIATION 

 Fig. 248. The effects of x-rays on the difTerentiation of the embryonic organs of Fundulus heteroclitus. 

 The X indicates the stage in development when the anlage of an organ appears. The line to the right 

 of each organ's name indicates over which period that organ may be affected by the irradiation (see 

 text). (Courtesy of Dr. A. N. Solberg and the editors of the Journal of Experimental Zoology.) 



which are the precursors of the affected 

 structures are particularly susceptible to the 

 treatments at that stage. Moreover, when 

 the same treatment is administered to em- 

 bryos at different stages not only will there 

 be a difference in incidence of a particular 

 anomaly but new anomalies may be pro- 

 duced; a new set of structures has become 

 susceptible. The data from Landauer and 

 Bliss ('46) and Landauer ('47a) in regard 

 to injection of insulin into chick embryos 

 illustrate this point (Fig. 247): when these 

 injections are made from to 64 hours of 

 incubation the most frequently encountered 

 anomaly is rumplessness. From 72 to 168 

 hours, however, the insulin produces a high 

 incidence of micromelia. 



There is a widespread belief that the 

 sensitive or critical period in the develop- 

 ment of a structure occurs chiefly during 

 its early formative stages, when the cells are 

 as yet undifferentiated or when the structure 



their formation, while others are affected 

 only before the structure appears. By draw- 

 ing a line perpendicular to the time axis 

 one can determine which structure will be 

 affected by the irradiation at any given stage. 

 This does not indicate the degree to which 

 the structures are affected. 



In order to obtain good anterior duplica- 

 tions by means of physical or chemical in- 

 terferences one must treat very early em- 

 bryonic stages. Only poorly organized or 

 posterior duplications have resulted from 

 centrifugation of amphibian blastulae and 

 gastrulae (Torrey and Breneman, '41; Pas- 

 teels, '47). In Stockard's ('21) experiments 

 with Fundulus anterior duplications were 

 obtained most successfvilly when early cleav- 

 age stages were treated. This was confirmed 

 by Hinrichs and Genther ('31), who demon- 

 strated that U-V irradiation 30 minutes prior 

 to first cleavage was most favorable for 

 production of anterior duplications. The pre- 



