REPRODUCTION. 41 
the yelk, so that the original disk which lay like a watch-glass 
on the rest of the ovum, has grown into a sphere. That portion ‘- 
of this area nearest the pellucid zone (area vasculosa) develops 
Fie. %5.—Diagrammatic transverse sections through a hypothetical mammal odsperm (Had- 
don). A. The yelk of the primitive mammalian odsperm is now lost. B. Later stage; 
the non-embryonic epiblast has grown over the embryonic area to form the coverin calls’ 
ep, epiblast of embryo ; ep’, epiblast of yelk-sac ; hy, primitive hypoblast ; y. s, yelk-sac, 
or blastodermic vesicle. 
blood-vessels that derive the food-supplies, which replenish the 
blood as it is exhausted, from the hypoblast of the area opaca. 
The first indications of future structural outlines in the 
embryo is the formation of the primitive streak, an opaque band 
in the long diameter of the pellu- 
cid area, opaque in consequence 
of cell accummulation in that re- 
gion. Very soon a groove (primi- 
tive groove) extends throughout 
this band, which gradually occu- 
pies a more central position. The 
relative thickness of: the several 
parts and the arrangement of cells 
may be gathered from Fig. 76. 
These structures are only tempo- 
rary, and those that replace them 
will be described subsequently. 
We have thus far spoken of 
cells as being arranged into epi- 
blast, hypoblast, and mesoblast. 
The origin of the first two has 
been sufficiently indicated. The i 
mesoblast forms.the intermediate me. 76.—surface view of pellucid area of 
‘ 7 . blastoderm of eighteen hours (Foster and 
germinal layer, and is derived Balfour). H, medullary folds; mc, med- 
from the primitive hypoblast,  WS8TY stoves Pr Primitive groove. 
which differentiates into a stratum of flattened cells, situated 
below the others, and constituting the later hypoblast, and in- 
