212 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
ance in the neighbourhood of the blastopore, between the 
hypoblast and the epiblast, though they are probably 
derived from the former. From this region they gradu- 
ally spread, first over the sternal, and then on to the 
tergal aspect of the embryo, and constitute the mesoblast. 
Epiblast, hypoblast, and mesoblast are at first alike 
constituted of nothing but nucleated cells, and they in- 
crease in dimensions by the continual fission and growth 
of these cells. The several layers become gradually 
modelled into the organs which they constitute, before 
the cells undergo any notable modification into tissues. 
A limb, for example, is, at first, a mere cellular out- 
growth, or bud, composed of an outer coat of epiblast 
with an inner core of mesoblast; and it is only subse- 
quently that its component cells are metamorphosed into 
well-defined epidermic and connective tissues, vessels and 
muscles. 
The embryo crayfish remains only a short while in 
the gastrula stage, as the blastopore soon closes up, and 
the archenteron takes the form of a sac, flattened out 
between the epiblast and the food-yelk, with which its 
cells are in close contact (fig. 57, C and D).* Indeed, as 
development proceeds, the cells of the hypoblast actually 
feed upon the substance of the food-yelk, and turn it to 
account for the general nutrition of the body. 
* Whether, as some observers state, the hypoblastic cells grow over 
and inclose the food-yelk or not, is a question that may be left open. I 
have not been able to satisfy myself of this fact. 
