REPRODUCTION. "7 
and, being filled with fluid, the zona is thinned and soon disap- 
pears, 
The germinal area alone is made up of three layers of cells 
(Fig. 104), the rest of the upper part of the odsperm being lined 
with epiblast and hypoblast, while the lower zone of the yelk- 
sac consists of epiblast only. 
Simple, non-vascular villi, serving to attach the embryo 
to the uterine walls, usually project from the epiblast of the 
subzonal membrane. In the rabbit they do not occur every- 
where, but only in that region of the epiblast beneath which the 
mesoblast does not extend, with the exception of a patch which 
soon appears and demarkates the site of the future placenta. 
The extension of the mesoblast takes place in every direction 
from the embryo except directly around the head; but the two 
Fic. 85.—Diagrammatic median vertical longitudinal sections through embryo rabbit (Had- 
don, after Van Beneden and Julin). A. Section through embryo of Fig. 84. B. Section 
through embryo of eleven days. al, allantois ; am, amnion ; a. ms, anterior median plate 
of mesoblast, formed by the junction of the anterior horns of the area opaca; a. pl, area 
placentalis ; a. v, area vasculosa ; ch, chorion ; cw, celom of embryo; ce’, extra-embry- 
onic portion of the body-cavity ; ep, epiblast ; hy, hypoblast ; m. unsplit mesoblast ; 0. a, 
orifice of amnion ; pl, placenta; pro.a, proamnion; s.t, sinus terminalis; v, epiblastic 
villi of blastodermic vesicle. 
expansions of the mesoblast which mark out this area extend 
for some distance in front of the head, and ultimately unite ; 
so that immediately in front of the head there is a circular 
region in which the blastoderm consists of epiblast and hypo- 
