148 CHARLES R. STOCKARD AND GEORGE N. PAPANICOLAOU 



much as ten days of development, there must certainly be a pre- 

 natal mortality of some extent previous to this time. Experi- 

 ments with the eggs of lower forms which develop outside of the 

 mother, permitting direct observation, speak for the great pre- 

 ponderance of an early embryonic mortality, many such eggs 

 dying during the cleavage and gastrular stages when subjected 

 to even slightly unfavorable conditions. We have some direct 

 evidence on 'early prenatal mortality' in female guinea-pigs 

 which have been examined by operation after repeated 'mating 

 failures.' The ovaries of some such animals contain corpora 

 lutea of pregnancy indicating that an embryo had been present 

 shortly before the examination. 



Pearl records that the eggs from alcoholized fowls are to a 

 high degree infertile. This he believes is due to many of the 

 germ cells as such having been killed by the treatment. By 

 infertile. Pearl means, of course, that no fertilization or zygote 

 formation took place, yet it is extremely difficult in all cases to 

 detect whether the early stages of development may not have 

 occurred and been followed by death and degeneration. The 

 death may have occurred during the cleavage or gastrular stages 

 while the egg was yet in the uterus of the hen and many of 

 the 'infertile eggs' might really be classed among the early pre- 

 natal mortalities. We make these suggestions merely as pos- 

 sibilities which to us are somewhat tempting, since if there was 

 actually an early prenatal mortality in some of these 'infertile 

 eggs' it would bring the effects of the alcohol treatment on the 

 fowls and mammals still closer together. It is only through our 

 recent analysis of the size of litters and mating failures, along 

 with careful examination of the pregnant females, that we have 

 become aware of the sometimes frequent very early embryonic 

 death. 



The second horizontal space shows the number of young from 

 the several lines that reached maturity, or lived over three 

 months. Here again the size of the litter is an important factor. 

 It may be stated generally that the power of survival of a guinea- 

 pig varies inversely with the size of the litter in which it is 

 born. We shall see beyond that this is also true of their birth 

 weight, growth rate, and certain other qualities so that in mak- 



