STRUCTURE AND DEVELOPMENTAL RATE 161 



of fins showed frequent peculiarities. Extreme cases arose in 

 which amorphous masses of embiyonic tissue were present on 

 the yolk, but no definite embryo was formed. There were simple 

 yolk-sacs with blood-cells and chromatophores scattered irregu- 

 larly through them. Along with these variously defective indi- 

 • viduals were almost invariably certain specimens which in gross 

 structural appearance were normal and succeeded in hatching 

 and swimming freely about. Others were almost normal with a 

 pulsating heart, but without a circulation of the blood. Further 

 more detailed conditions need not be mentioned. 



This list of defects is sufficient to show that the types and actual 

 individual conditions resulting from a simple interruption of 

 development by reducing the temperature are all identical in 

 character with those induced by treating Fundulus eggs with 

 various chemical solutions (Stockard, '07, '09, '10, '15, etc.) dur- 

 ing their early developmental stages or actually with the results 

 of certain mechanical operations upon these (Lewis, '09) and 

 other eggs (Stockard, '13). Furthermore, deformed hybrids 

 resulting from crosses between distantly related species also 

 present exactly the same structural peculiarities (Newman, '15). 

 And finally the progeny derived from male guinea-pigs that have 

 been chemically treated for long periods of time occasionally 

 exhibit exactly similar deformities of their eyes and other parts 

 (Stockard, '13; Stockard and Papanicolaou, '15 and '17). 



It seems difficult to imagine that the deformities occurring 

 among the eggs that have been merely interrupted by being 

 placed in a refrigerator temperature could be interpreted as other 

 than simple arrests in development resulting from the slow prog- 

 ress which had taken place at certain critical times. It seems 

 equally as certain that the comparable conditions following the 

 other experimental procedures have resulted from a similar cause, 

 simply a lowering of the developmental rates of certain parts at 

 critical moments in their origin or developmental history. In 

 the several sections to follow I shall give much crucial evidence 

 bearing on such an interpretation. 



