16 ANIMAL PHYSIOLOGY. 
the placental sac by Duval, who has recently described this pro- 
cess. Villi, or tubular vascular outgrowths, spring from the 
lining of this sac and serve to convey the absorbed and prob- 
ably altered albumen to the embryo, in which process of vas- 
cular transport of nourishment the yelk-sac, that also abounds 
in blood-vessels as well as the allantois, takes part. The 
physiological import of the various structures above described 
will be considered more fully later. At this point a compari- 
son of the formation of the corresponding parts in mammals 
will be undertaken. 
Tue FaraL (EMBRYONIC) MEMBRANES OF MAMMALS. 
The differences between the development of the egg mem- 
branes of mammals and birds are chiefly such as result from 
Fie. 83. Fie. 84. 
Fic. 83.—Diagrammatic ee section of odsperm of rabbit at an advanced stage of 
pregnancy (K6lliker, after Bischoff). a, amnion; al, allantois with its blood-vessels ; e, 
embryo ; ds, yelk-sac ; ed, ed’, ed’’, bypoblastic tii eae of the yelk-sac and its stalk 
(umbilical vesicle and cord); fd, vascular mesoblastic membrane of the umbilical cord 
and vesicle ; , placental villi formed by the allantois and subzonal membrane ; 7, space 
filled with fluid between the amnion, the allantois, and the yelk-sac ; st, sinus terminalis 
(marginal vitelline blood-vessel) ; 2, urachus, or stalk of the allantois. 
Fic. 84.—Diagrammatic dorsal view of an embryo rabbit with its membranes at the stage of 
nine somites (Haddon, after Van Beneden and Julin). al, allantois, showing from behind 
the tail fold of the embryo ; am, anterior border of true amnion; a. v, area vasculosa, the 
outer border of which indicates the farthest extension of the mesoblast ; bl, blastoderm, 
here consisting only of epiblast and hypoblast ; 0. m. v, omphalo-mesenteric or vitelline 
bhatt iP. ate proamnion ; pl, non-vascular epiblastic villi of the future placenta; s. t, si- 
nus terminalis, 
the absence in the former of an egg-shell and its membranes, 
and of yelk and albumen. The mammalian ovum is inclosed 
by a zona radiata (zona pellucida) surrounding another very 
delicate covering (Fig. 58). 
The growth of the blastodermic vesicle (yelk-sac) is rapid, 
