378 The American Naturalist. [April, 
the insertion of the mesometrium. In Erinaceus the placenta fæœtalis is 
affixed to the side of the lumen of the uterus diametrically opposite to 
the insertion of the mesometrium. The COnclusion is therefore forced 
upon us that there is no exact homology between the maternal portion of 
the placenta in the hedge-hog and that of the same part in the large 
majority of other mammalian types. The Name “ trophoblast’’ which 
Hubrecht proposes for the ‘“ outer layer’’ of the blastocyst is exceed- 
ingly apt and convenient, while his elaborate studies as to the role it 
plays in the formation of the placenta, as well as its growth and fate, 
constitute a most valuable contribution to the embryology of the higher 
vertebrates. Nevertheless, one cannot help regretting that the obvious 
and clear homology of this layer with the serous envelope, subzonal 
membrane,—Deckschicht as this layer has been variously called,—has 
not been more strongly emphasized. Of such a homology there cannot 
be the sliglitest doubt ; the only difficulty in making it out is due to 
the excessive concentration or abbreyiation of the early stages of 
development already referred to. The Modification of their early 
stages and their abbreviation in mammalia are also clearly adaptive 
and directly so under the influence of trophic stimuli, which differ very 
widely in character in the different mammalian orders. These differ- 
ences are apparently due to the effects of what may, for want of a 
better phrase, be called the reciprocal trophic stimuli exerted recipro- 
cally upon each other by the blastocyst and uterine walls jn the differ- 
ent types during the initial stages of development. The variations in 
the differentiation and arrangement of the mucosa and its vessels in 
the different forms must have had something to do with the genesis of 
such different methods of differentiation of the primary stages of 
mammalian development. The expectation of ever unravelling thef 
causes of such differences through a study of the early development of 
the fœtus alone will be fruitless. The Processes are in the clearest 
possible manner directly adaptive in certain yery definite ways, which 
purely morphological study is utterly and forever incapable of explain- 
ing, and is no less irrational and absurd than to attribute such modifi- 
cation to the “action ” of natural selection. Joun A. RYDER. - 
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