56 LIBBIE H. HYMAN. 



the Fundulus embryo is most affected by reagents when the germ 

 ring is near the equator of the eggs and yields the maximum num- 

 ber of teratological forms at this period. Newman ('15) found 

 that in heterogenic fish hybrids development very frequently stops 

 at this stage. 



V. RELATION OF THE GRADIENTS TO TERATOLOGICAL 



DEVELOPMENT. 



The literature on the structure, occurrence, and experimental 

 production of teratological vertebrate embryos has now attained 

 such vast proportions that an adequate review of it would be a 

 huge task. I shall here consider only the experimental produc- 

 tion of terata among teleosts. The discussion applies, however, 

 to vertebrate terata in general, since the mode of development is 

 much the same throughout the vertebrates. 



Morgan ('95) found that if the developing eggs of Tauto- 

 golabrus are placed in diluted sea-water, the development of the 

 anterior end of the embryo is commonly inhibited ; the medullary 

 folds fail to close and the anterior end remains flattened out on 

 the yolk. In one case the formation of the embryo was com- 

 pletely inhibited but the germ ring continued to develop and close 

 in normal fashion. 



The experiments of Stockard ('06, '07, '09, '10) on the produc- 

 tion of teratological forms in Fundulus are familiar to every one. 

 His first experiments were performed with lithium chloride. In 

 the stronger solutions, the eggs either cease to develop in an early 

 blastoderm stage or else very abnormal embryos are produced 

 with poorly developed anterior and posterior ends, short bodies, 

 and no eyes. The trunk and auditory vesicles are, however, 

 present. In weaker solutions of lithium chloride, the embryos 

 are less abnormal. The expansion of the blastoderm over the 

 yolk may be retarded resulting in spina bifida. The embryos are 

 commonly short with no eyes or defective eyes. The developing 

 eggs were found to be most sensitive to the treatment between 18 

 and 22 hours after fertilization when the germ ring is near the 

 equator of the egg. In later experiments Stockard found that 

 a number of substances would produce the same effects on the 



