1899. ] Embryology. 377 
to question the primitive position in many features, especially so far 
as adult characters are concerned, of the hedge-hog and its insectiv- 
orous allies, the assumption that its placentation is primitive is a very 
different matter, While no one can help but admire the wonderful 
fidelity and care with which the facts of placental development are 
recorded, since the plates for histological details are simply unrivalled, 
the conclusion that the placentation of the hedge-hog is primitive is 
far from warranted. 
So far from Prof. Hubrecht’s assumption as to the primitive nature 
of this type’s placenta being borne out by his facts, it is distinctly and 
emphatically negatived by them. In the first place, a ‘‘ reflexa’’ such 
as is described by him is found in comparatively few forms. More- 
over, such a development of the uterine mucosa is distinct evidence 
in favor of the conclusion that the placenta in forms having such a 
“ reflexa” is specialized. In some rodents, man, possibly Tamandua, 
also Erinaceus, Talpide, Rhynchocyon, in all of which it is pretty 
certain that the whole complex series of primary differentiations of the 
blastocyst or blastodermic vesicle are completed without the accom- 
paniment of an excessively rapid growth in its size, such as occurs in 
the rabbit and opossum, where also there is either no reflexa formed, 
or traces only of such an organ are developed later. In the first- 
named forms there has occurred an adaptive abbreviation of the early 
processes of development, which have not supervened in the last two, 
or in the rabbit and opossum. 
The peculiar mode of development of the hypoblast in the hedge- 
hog is again specialized and widely different from what it is in most 
rodents. 
In the same way the site of the placenta and the germinal area are 
different from those of other types, and therefore specialized. In the 
first place, the embryo itself is formed at a point in the blastodermic 
vesicle which is exactly opposite its homologue in the rabbit, mouse, 
and rat, and probably even man, Bradypus, and Tamandua, as well as 
many carnivora. In these last-named it is formed in a dorsal position 
in the uterine lumen or just benéath the insertion of the mesometrium. 
In Erinaceus the embryo is formed at a point on the surface of the 
blastocyst diametrically opposite to the point of insertion of the meso- 
metrium. The embryo in the first-named series therefore has its 
dorsal aspect coincident at first with that of the parent ; in the hedge- 
hog that aspect coincides with the ventral aspect of the parent. The 
site of the attachment of the placenta is similarly reversed. In the 
majority of forms the position of the placenta is immediately beneath 
