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. 



