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 sun-ounding 

 conditions. 



Without entering into details which lie b,eyond 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 tbe carapace. 



