THE EARLY DEVELOPMEXT OF THE MAESUPIAIJA. 113 



vestigial representative of the shell of the presumed oviparous 

 common ancestor of the Metatheria and Eutheria. The 

 Eutherian ovum, on the other hand, has lost all trace of the 

 shell in correlation with its more complete adaptation to the con- 

 ditions of iiitra-uterine developmeut. The albumen hyev is 

 variable in its occurrence, being present in some (e.g. rabbit) 

 and absent in others (e.g. pig, Assheton), whilst the zona 

 itself, though always present, is variable both as to its thick- 

 ness and the length of time it persists. 



Strangely enough, although the prevaling opinion amongst 

 mammalian embryologists is that the Eutherian ovum has 

 been derived phylogenetically from an egg of the same telo- 

 lecithal and shell-bearing type as is found in the Monotremes, 

 no one, so far as I am aware, has ever taken the shell into 

 account, and ventured to consider in what way its total dis- 

 appearance from an ovum already greatly reduced in size, 

 might affect the course of the early developmental phenomena. 

 That is what I propose to do here, for in my view it is just in 

 the complete loss of the shell by the Eutherian ovum that we 

 find the key to the explanation of those remarkable differences 

 which nre observable between the early ontogeny of the 

 Eutheria and Metatheria, and which culminate in the entypic 

 condition so distinctive of the former. The acquisition of a 

 shell by the Proamniota conditioned the appearance of the 

 amnion. The loss of the shell in the Eutheria conditioned the 

 occurrence in their ontogeny of entypy. 



As we have seen, the mammalian ovum, already in the 

 Monotremes greatly i-educed in size as compared with that of 

 reptiles, and quite minute in the Metatheria and Eutheria, 

 contains within itself neither the cubic capacity nor the food 

 material necessary for the production of an embryo on the 

 ancestral reptilian lines. We accordingly find that the 

 primary object of the first developmental processes in the 

 mammals has come to be the formation of a vesicle with a 

 complete cellular wall, capable of absorbing nutrient fluid from 

 the maternal uterus and of growing rapidly, so as to provide 

 the space necessary for embryonal differentiation. 



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