210 EDWARD MCCRADY, JR. 



tapetum lucidum of the eye is in the pigment layer instead of in the 

 chorioid, and is composed of crystals of guanin instead of fibrous 

 cells. This is like the condition in the crocodile family, and in con- 

 trast to the condition in the ungulates and carnivores (private com- 

 munication from G. L. Walls). The subintestinal vessel is a vein as 

 in lower vertebrates, not an artery as in all placental mammals 

 (Kimball, '28). The capillaries of the cerebral cortex form simple 

 loops between arterioles and venules instead of being plexiform 

 as in higher mammals. This is like the condition in amphibians and 

 reptiles (Wislocki, '37). 



It is difficult to imagine that all or even a large proportion of these 

 features are the result of degeneration from a eutherian condition 

 It seems much more reasonable to interpret them as evidence that the 

 marsupials have diverged less from the original common stock than 

 have the eutheria. Everything seems to bring us back to Huxley's 

 original theory essentially unscathed. The only modification of it 

 which seems possible is the one advocated by Wilson and Hill, and 

 Flynn that the metatheria acquired an allantoplacenta before they 

 gave rise to the marsupials and the eutheria. This is possible; but 

 neither compelling, nor, I think, likely. 



One point on which almost all the evidence is in agreement is that 

 the Didelphyidae are the most primitive marsupials alive today, and 

 that fact adds another source of interest to the embryology of the 

 .Virginia opossum. 



BIBLIOGRAPHY 



In response to numerous requests from this country and abroad for an opossum 

 bibliography, it has been considered desirable to bring together at this place a 

 rather comprehensive list which may be of wider interest than the volume to 

 which it is attached. The references listed below, therefore, are not all cited in 

 the text. Many deal with subjects other than embryology. A few are concerned 

 with related animals, not with the opossum itself. Fewer yet refer to animals 

 not even closely related, but yielding data which throw light upon some observa- 

 tion or principle referred to in the text. 



The original list compiled by the author included a much larger number 

 of titles. It has been contracted to the present size by the elimination of those 

 which could not be consulted on account of inaccessibility of the journal, or 

 incompleteness or incorrectness of the reference data ; those which deal with 

 distantly related marsupials a knowledge of which is not very pertinent to 

 anything discussed in the text; and finally, a few which are apropos in subject 

 matter, but so superficial or incorrect as to be considered more disconcerting than 

 useful. This last category of eliminations might have been made more extensive, 

 but the author was anxious to err in the direction of over-comprehensiveness rather 

 than of unduly personal selectiveness. 



