STUDIES IN MAMMALIAN EMBRYOLOGY. 283 
Studies in Mammalian Embryology. 
I.—The Placentation of Erinaceus Europeus, 
with Remarks on the Phylogeny of the Pla- 
centa. 
By 
A. A* W. Hubrecht, LL.D., C.M.Z.S. 
Professor of Zoology in the University of Utrecht. 
With Plates XV to XXVII. 
INTRODUCTION. 
Tue observations which are recorded in the following pages 
were commenced five years ago with the definite object of at- 
tempting a step towards the solution of certain problems which 
are still left open, and of numerous contradictions which are 
encountered in the field of the embryological development of 
the Mammalia. For this reason it appeared to be a peremptory 
condition that among the Mammalia monodelphia an 
order should be selected which has retained more than other 
orders the characters of the primitive stock from which 
the other more specialised mammalian orders have been 
derived. 
In this order the species which might be said to be the least 
differentiated in any special direction would again be the more 
favorable starting-point for comparative research. Huxley’s 
remarkable article in the ‘ Proceedings of the Zoological 
Society ’ for 1880 (“ An Application of the Laws of Evolution 
to the Arrangement of the Vertebrata, and more particularly 
of the Mammalia,” p. 649) has thrown new light on the pro- 
