14 THE PHYSIOLOGY OF TWINNING 



mental rate at all stages. It seems obvious, however, 

 that though foreign sperm less completely stimulates 

 the egg than own sperm, yet some stimulus is given, 

 for hybrid embryos develop more rapidly and more 

 normally than do parthenogenetic embryos. 



C. Twins due to crowding. — Varying percentages of 

 twins of an even more advanced type were frequently 

 found in control batches of eggs that had been fertilized 

 by own sperm, but had been allowed to remain in much 

 too large numbers in relatively small containers. Some 

 of the most advanced twin larvae were discovered 

 under these conditions, and such twin larvae, when 

 transferred to new quarters, where they had plenty of 

 room and oxygen, developed farther than any other 

 twin larvae observed. In some cases they practically 

 recovered the single condition through the absorption of 

 the smaller twin by the larger. It seems obvious that 

 these larvae in the crowded cultures had suffered a 

 period of growth depression due to the low oxygen and 

 high CO2 content of the sea water. It was my intention 

 to perform some experiments involving lack of oxygen, 

 but the work was interrupted before there was oppor- 

 tunity to do so. 



Retarded development, the primary cause of twinning. — • 

 In all three of these methods of twinning the common 

 factor is undoubtedly a retardation, more or less severe, 

 of the normal rate of development. This was not at all 

 an unlooked-for result; quite the contrary. One who, 

 like the writer, had for years realized the fact that most, 

 if not all developmental anomalies, are, in last analysis, 

 the result of interferences with the normal developmental 

 rate of organisms, and who had offered a theory of arma- 



