28 



Methods and Techniques 



For the embryologist who considers him- 

 self primarily concerned with descriptive 

 and morphological questions, the problems 

 arising from these examples, and the count- 

 less others that could have been enumerated, 

 may seem of little moment. He can evade 

 them by ignoring them, or more profitably, 

 he can meet them by specifying as strictly 

 as possible the conditions under which he is 

 making his observations. For the investiga- 

 tor, however, who is looking for "normal" 

 controls for experimental material, the situ- 

 ation becomes more critical. 



It becomes especially so for the inter- 

 pretation of certain isolation experiments 

 which will be discussed in more detail be- 

 low, and it presents very particular problems 

 in the case of some transplantation experi- 

 ments. The rate of growth of an eye or a 

 limb grafted heteroplastically onto an am- 

 phibian host varies according to whether or 

 not the host is maximally or less than maxi- 

 mally fed (Twitty and Schwind, '31); how 

 often does an Amblystoma in nature enjoy 

 a condition of maximum repletion? The 

 growth of an Amblystoma embryo varies ac- 

 cording to the organisms constituting the 

 diet it is fed; who can define the "nor- 

 mal" diet of Amblystoma in nature? Mem- 

 bers of the same species of Amblystoma 

 reared under similar laboratory conditions 

 develop at different rates of growth when 

 collected in Pennsylvania and New Jersey 

 on the one hand, in Illinois on the other 

 (DuShane and Hutchinson, '44); which is 

 the more "normal" larva, that collected in 

 the East or that which is spawned in the 

 Midwest? This is a more than philosophical 

 problem when experimentalists in Princeton, 

 for instance, are comparing their results with 

 those of investigators working at the Uni- 

 versity of Chicago in an attempt to work out 

 basic mechanisms of development. And 

 while it is particularly accentuated by the 

 results of the heteroplastic experiments, it 

 touches the heart of each investigation in- 

 volving the growth of the whole or parts of 

 the developing amphibian. 



In the case of more complicated experi- 

 ments it raises even more complex issues. 

 Haploid tissues developing from the hybrid 

 androgenetic merogons of Hadorn ('34, '37) 

 and Baltzer ('40), for instance, die in the 

 embryos which they constitute but survive 

 in tissue culture or after transplantation to 

 normal diploid hosts of one of the experi- 

 mental species. Is a favorable tissue culture 

 medium or the tissues of a diploid host to be 

 defined as more "normal" or "abnormal" for 



these cells than the haploid parent embryo 

 from which they were derived? Under which 

 conditions are the operations of the gene in 

 development more "normal"? In other cases, 

 what seems "optimvim" according to the sub- 

 jective judgment of an investigator con- 

 cerned with a particular experiment may 

 lack biological significance to the organism. 

 Of what biological significance is it to Lyt- 

 echinus that its egg can be so treated in the 

 laboratory that it is more readily activated 

 by sperm of a foreign species than by that 

 of its own (Tennent, '25)? Tennent ('10) 

 has fovmd that in reciprocal crosses of Hip- 

 ponoe X Toxopneustes the larvae were of 

 the Hipponoe type when raised in sea water 

 of higher pH, of the Toxopneustes type when 

 reared in sea water of lower pH, and has 

 suggested that seasonal variations in hybrid 

 echinoderm larvae obtained in other labora- 

 tories might be accounted for by seasonal 

 variations in the alkalinity of the seas; who 

 is to say whether autumn or spring is more 

 "normal" for echinoderm hybrids? Under 

 which conditions, and this is the crux of the 

 matter, does the action of the gene, which 

 according to its end-effect varies in the dif- 

 ferent situations, more closely simulate its 

 norm? 



This is not the appropriate place to take 

 up the developmental action of genes, which 

 will be considered in a separate section be- 

 low; but it is necessary to emphasize here 

 the advantages to the embryologist of work- 

 ing with genetically known and genetically 

 specified material when he is able to do so. 

 Embryos vary. Many of the factors which 

 induce them to vary are difficult to control, 

 as we have seen. The genetic factors are per- 

 haps uncontrollable too when specimens are 

 collected in nature. But when the genetic 

 factors are known, they can be controlled, 

 and thereby great strides into unknown ter- 

 ritory can be accomplished (cf. Gluecksohn- 

 Schoenheimer, '49), and, as important, the 

 results can be specified in the best biological 

 sense of the word. 



Consideration, in fact, of all the limitations 

 to interpretation discussed above, and of 

 many others like them, leads to the same in- 

 escapable necessity for specification of the 

 conditions under which an investigation pro- 

 ceeds. It may often accrue to the advantage 

 of the investigation, rather than otherwise, 

 that observations and experiments carried 

 out in different situations lead to different 

 outcomes as well as different interpreta- 

 tions. Only when investigators specify as 

 closely as possible the conditions against 



