904 General Notes. | September, 
phylum embraces the eutherian type, or the viviparous Mamma- 
lia, in which the ovum is invested only by a zona radiata. The 
ovum also becomes more or less covered over by reflections of 
the uterine epithelium, or processes from the surface of the ovum 
are received into furrows or pits of the mucous surface of the 
uterine cavity, in which embryonic development proceeds more 
or less nearly to completion, and where the developing embryo 
acquires nourishment after the manner of a parasitic organism. 
The cleavage of the egg is total, and leads to the formation of a 
hollow, rapidly-enlarging, blastodermic vesicle or blastula, in 
which no vitelline matter or deutoplasm lies free in the ventral 
pole or yolk sack. This group is characterized by its viviparity, 
the development of a placenta and the possession of a functional 
uterus; it has obviously descended from one of the preceding 
groups which had ova provided with a large yolk, the yolk hav- 
ing gradually atrophied or failed to develop as the peculiar vivip- 
arous mode of development became more and more pronounced. 
. a, The lowest division of the Eutheria would be separated 
from the highest, on embryological grounds, by the circumstance 
that the vascular system of the foetus is brought into relation with 
the maternal vessels by means of the vessels of the yolk sack 
(Owen, Osborn). Or by means of pseudopodal processes of the 
cells forming the yolk sack, Phascolarctos (Caldwell). Allantois 
more or less rudimentary. Didelphia. 
6. In the highest division of the Eutheria the vascular system 
of the foetus is brought into relation with the maternal vessels by 
means of the allantois, which is concerned in forming a villous or 
spongy vascular mass known as the placenta. Monodelphia. 
In the present state of our knowledge it seems premature to 
i attempt a definition of the orders of the Monodelphia upon the 
-basis of placental characters alone, as the following scheme seems- 
to show. It is difficult, however, not to believe that the non- 
allantois invests the whole embryo. In this succession the orders 
of the Monodelphia will therefore be here arranged as nearly as 
possible : 
aa. Placenta non-deciduate, diffuse or cotyledonary. Cetacea, 
Sirenia, Ungulata. 
_ 66, Placenta non-deciduate, diffuse (Manis); or non-deciduate ? 
early zonary (Orycteropus); or non-deciduate, zonary (Dasypus 
vemcinctus) ; or deciduate discoidal (Dasypus sp., Cholcepus). 
a deciduate, zonary. Carnivora, Proboscidea, Hyra- 
