96 THE PHYSIOLOGY OF TWINNING 



tunnel several inches deep, enlarged into a flask-like 

 chamber at the bottom. The incubation of the eggs 

 depends entirely upon the heat of the sun that may 

 penetrate the soil. The breeding season comes either in 

 May or early June and it is sometimes decidedly cold 

 and sunless during a considerable part of the period of 

 incubation. There can be no doubt that climatic irregu- 

 larities have a very marked effect upon the development 

 of chelonian eggs in north temperate latitudes. I have 

 examined a great many nests and have found whole 

 batches of eggs dead and decaying, probably killed by a 

 cold spell during the early periods of incubation. In 

 other batches of eggs I have found a very large percentage 

 of embryos abnormal in various respects: some with 

 imperfect eyes; some with heads small and irregular; 

 some with one or more feet lacking or tail lacking; some 

 with deformed carapace; many with irregularities of the 

 scute pattern of both carapace and plastron, and associ- 

 ated abnormalities. All of these irregularities are 

 obviously due to unfortunate developmental condi- 

 tions — ^probably low temperatures. In several hundred 

 embryos of various species of turtles examined I have 

 never found a case of twins or even of unmistakable 

 double monstrosity. One type of abnormahty, however, 

 that was fairly common was a condition of more or less 

 extensive doubling of the median series of scutes on the 

 carapace. This type of irregularity was found to be 

 closely correlated with a similar doubling of neural 

 plates, which are the broadened dorsal spines of the 

 vertebrae. Not infrequently there was a dichotomous 

 fission of a rib in association with such doubling, and 

 when there are twinned ribs there are usually twinned 



