Teratogenesis 



indicate some of the sources of confusion in 

 this type of experimentation. 



CONDITION OF GAMETES AND 

 ZYGOTES 



One of the most consistent observations 

 made by investigators in the field of experi- 

 mental teratology is the extreme variability 

 in the reactions of fertilized eggs or embryos 

 to the treatments to which they are subjected. 

 Zygotes from the same set of parents do 

 not all respond equally to a given set of 

 conditions, and the incidence of response 

 differs in offspring from different parents. 

 Variations in reactivity of zygotes and em- 

 bryos may be due to alterations in their 

 physiological condition or to their genetic 

 constitution. 



Physiological Condition. It is quite evident 

 that normal development can be expected 

 only from zygotes that have been produced 

 and raised tmder normal conditions. Some 

 effects may be prodviced by factors acting 

 before the onset of embryonic development. 

 The experiments of Witschi ('22, '30, '34) 

 on eggs which were retained in the female's 

 uterus for 3 to 5 days prior to fertilization 

 are familiar examples. Such over-ripe eggs 

 are frequently polyspermic. Among eggs fer- 

 tilized by one sperm Witschi found axial 

 duplications, supernumerary appendages, 

 pigment changes and a tendency towards the 

 prodviction of neoplasms. According to Wit- 

 schi such abnormalities result from a lack 

 of "coordination" in the egg and embryo, 

 probably due to alterations in the cortical 

 layer. In chick eggs it has been known for 

 a long time that proper storage conditions 

 are essential for good development (Landauer, 

 '51). Proper temperatvire and humidity must 

 be maintained. Extended storage, even under 

 optimum conditions, may also result in im- 

 paired development. Poor pre-incubation 

 conditions result in poor hatchability, early 

 mortality and anomalies. In experiments in 

 which the effects of some particular treat- 

 ment of eggs are to be evaluated these con- 

 ditions must be controlled carefully. The 

 reactions of eggs which have been im- 

 properly stored cannot be compared with 

 those which have been handled carefully. It 

 is likely that at least some of the contradic- 

 tions in the literature are due to these factors. 

 Even under proper conditions of storage one 

 may encounter differences in susceptibility 

 of eggs from the same hens at different times 

 of the year. Landauer and Baumann ('43) 

 foimd that the incidence of rumplessness 



711 



resulting from mechanical jarring of eggs 

 was greater during the summer months than 

 in the spring. In another study Landauer 

 ('43) found that there was a greater in- 

 cidence of micromelia during the fall and 

 early winter than in the late winter and 

 early spring. The reverse was found for eye 

 defects (micro- and anophthalmia). These 

 few illustrations are presented to indicate 

 that one must know the condition of material 

 with which one performs experiments. There 

 is little doubt that similar variations in 

 gametes and zygotes exist in other forms. 



The researches of the Hertwigs and others 

 (see Frets, '31, for bibliography) demonstrate 

 that the fertilization of eggs with sperm 

 previously subjected to treatment with var- 

 ious chemicals (methylene blue, chloral hy- 

 drate, strychnine, etc.) results in the forma- 

 tion of many terata. Similar results may be 

 obtained after irradiation [it is important to 

 note that after exposure of sperm to increased 

 doses the incidence of abnormalities decreases 

 because of the ability of the otherwise in- 

 activated sperm to initiate parthenogenetic 

 development (Hertwig, '28; Rugh and Exner, 

 '40) ] . More clearly related to the practical 

 consideration of physiological conditions are 

 observations on the effects of fertilization of 

 eggs by "stale" sperm. Nalbandov and Card 

 ('43) have obtained data of this sort for 

 chickens. They removed the males from pens 

 of tested hens and collected eggs for as long 

 as 35 days following this separation. They 

 found that eggs fertilized with older sperm 

 had a lower hatchability and greater early 

 embryonic mortality, with embryonic death 

 occurring earliest in those eggs fertilized by 

 the oldest sperm. There was no increase in 

 incidence of terata. Aberrant development 

 was found in a similar experiment (Dhar- 

 marajan, '50). Some observations (unpub- 

 lished) made by the writer at Storrs indicate 

 that embryos from eggs fertilized by stale 

 sperm are much more svisceptible to various 

 treatments (both in regard to mortality and 

 incidence of abnormalities) than are more 

 vigorous embryos. 



Genetic Constitution. The importance of 

 genetic uniformity in experimental material 

 is well established at the present time. There 

 is still, however, a tendency among some 

 investigators to neglect this factor in com- 

 paring their results with teratological agents 

 with those of others. Stockard emphasized 

 this point by indicating the difference be- 

 tween trout and Fundulus embryos in regard 

 to the incidence of twinning. Trout embryos 

 not only yielded a much higher percentage of 



