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Attachment of the embryo. That the rabbit embryo is 
attached to the surface of the uterus by a thickened region (area pla- 
centalis) of the ectoderm of the germinative area was first shown by 
VAN BENEDEN and JULIN (1); this discovery has since been confirmed 
by Minor (7), Masıus (6), Duvat (3), and others. That a similar 
method of attachment exists in other mammals has been shown by 
STRAHL (10, 11); in the dog it has been recorded by G. HEINRICIUS (4). 
In all these cases the thickened ectoderm is found to be closely ad- 
herent to the uterine surface upon which it is apt to remain when the 
ovum is forcibly removed; it fits exactly to the surface of the maternal 
epithelium. There is no visible layer of cement, and we do not know 
how the adherence is made so close. 
It is probable that we have here the primitive form of attachment, 
and that therefore the evolution of placenta began with the differentiation 
of the ectoderm of the area placentalis. 
There is another type of attachment found in the hedgehog and 
in the rodent ova with inversion of the germ-layers, characterized by 
the ovum being so closely invested by the uterine mucosa that the 
whole surface of the ovum comes in contact with the maternal tissues, 
see E. SELENKA (9) and HUBRECHT’s superb monograph on the pla- 
centa of the hedgehog (5). 
Degeneration of uterine tissues. Over the region of the 
placental attachment, which varies in different animals as to position, 
there occurs an extensive degeneration of the tissues of the uterine 
mucosa, affecting both the covering epithelium, the glands and the 
connective tissue. The degeneration takes place most rapidly in the 
epithelium and glands, while the connective tissue undergoes an exten- 
sive hypertrophic metamorphosis usually in the form of the develop- 
ment of decidual cells, before the degenerative change acquires the 
upper hand. The nature and extent of the degenerative changes have 
become known for various types by investigations published since 1888, 
several of which appeared during 1889 (Minor (7), Mastus (6), HEIN- 
RICIUS (4), Duvau (3), HUBRECHT (5), STRAHL (10) etc.), and which 
represent simultaneous and independent researches. In view of what 
we now know it must be considered probable that in all placental 
mammals, or at least in the orders of the unguiculate series, the 
uterine degeneration is an invariable factor in the development of the 
placenta. 
The form of degeneration is not fixed, but varies greatly. This 
is illustrated by the histories of the decidua in man and in the rabbit. 
Other modifications occur in the dog, the hedgehog, the mole, and 
doubtless in other animals. 
