222 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
impregnated ovum, with the conditions to which it is 
exposed; just as the forms evolved from a crystallising 
fluid are dependent upon the chemical composition of 
the dissolved matter and the influence of surrounding 
conditions. 
Without entering into details which lie beyond the 
scope of the present work, something must be said re- 
specting the manner in which the complicated internal 
organisation of the crayfish is evolved from the cellular 
double sac of the gastrula stage. 
It has been seen that the fore-gut is at first an insig- 
nificant tubular involution of the epiblast in the region 
of the mouth. It is, in fact, a part of the epiblast turned 
inwards, and the cells of which it is composed secrete a 
thin cuticular layer, as do those of the rest of the epi- 
blast, which gives rise to the ectodermal or epidermic 
part of the integument. As the embryo grows, the fore- 
gut enlarges much faster than the mid-gut, increasing 
in height and from before backwards, while its side-walls 
remain parallel, and are separated by only a narrow 
cavity. At length, it takes on the shape of a triangular 
bag (fig. 57, D, fg), attached by its narrow end around 
the mouth and immersed in the food-yelk, which it 
gradually divides into two lobes, one on the right and one 
on the left side. At the same time a vertical plate of 
mesoblastic tissue, from which the great anterior and 
posterior muscles are eventually developed, connects it 
with the roof and with the front wall of the carapace. 
