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 craj^fish 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. 



