Amphibians 



The following chapters will deal in detail 

 with these relationships between the germ 

 layers. We merely draw the general conclu- 

 sion that the organizing capacity resides pri- 

 marily in the entomesoderm and not in the 

 ectoderm. The former consists of regionally 

 different tissue components which determine 



237 



may cause various degrees of cyclopia and 

 microcephaly: hybridization (Moore, '46); 

 parthenogenesis, androgenesis or over-ripe- 

 ness of the egg (Rvigh, '48); low or high 

 temperatures (Atlas, '35; Hoadley, '38); 

 centrifugation (Jenkinson, '14; Motomura, 

 '31; Pasteels, '40; Pasqviini, '42); irradiation 



J^ 



i^^' 



\r' 



r 



W" ^^ ^- 



Fig. 79. Varying degrees of hypomorphism in Rana pipiens, produced by brief alkali treatment at the 

 early gastrula stage. All six malformations are of the same age. Note the correlation between the extent of 

 invagination and the degree of ectodermal organization. In a, the three germ layers have remained side by 

 side; d possesses suckers and a single eye; / shows spina bifida and a double tail. (Original.) 



or "induce" the ectoderm to enter into its 

 various kinds of differentiations. To insure 

 a response, intimate contact between ecto- 

 derm and the subjacent "inductor" is neces- 

 sary. 



VARIOUS AGENTS CAUSING 

 SUBNORMAL ORGAN PATTERNS 



Hypomorphic forms corresponding to par- 

 tial exogastrulae occur "spontaneously" in 

 all vertebrates (Schwalbe, '07; Wolff, '48) 

 and they have been produced experimentally 

 by a great diversity of procedures (see Sec- 

 tion XIV, on teratogenesis). Confining our- 

 selves to the amphibians and omitting further 

 details, we list some of the procedures which 



of the spermatozoa (Rugh, '39); crowded 

 conditions, alkali (Holtfreter, '44c, '48b); fat 

 solvents, acids, and a variety of salts, espe- 

 cially lithium chloride (Jenkinson, '06; Leh- 

 mann, '33, '38; Adelmann, '34, '36; Copen- 

 haver and Detwiler, '41; Pasteels, '45). 



Thus a great number of unrelated physical 

 and chemical interferences from the outside 

 as well as intrinsic genetic changes can affect 

 development in such a way that the resulting 

 malformations are strikingly similar. Not 

 all of these experimental data have been 

 sufficiently analyzed to decide when, where 

 and by what mechanism the different agents 

 have deranged the developmental processes. 

 However, it can be safely assvimed that in 

 all these instances the pattern of the mal- 



