204 BIOLOGY: GENERAL AND MEDICAL 



cellular layers until the original bell-shape gives place to 

 a more spheroidal form with a central opening, the 

 blastopore. 



Embryos at this stage bear a distinct resemblance to 

 certain larvae of ccelenterates, and indeed this diploblastic 

 larva is the general plan of development, as well as the 

 foundation of structure of the medusas. 



Holoblastic eggs of still higher animals next progress 

 to the formation of triploblastic larvae through the forma- 

 tion of a third cellular layer, the mesoblast, which arises 

 from two rudiments symmetrically arranged on opposite 

 sides of the central axis. In different embryos its 



FIG. 86. Section through the germ disc of a freshly laid unfertilized hen's egg. 

 fh, cleavage-cavity; wd, white yolk; vw, lower cell layer; dw, upper cell layer 

 of the blastula. (After Duval.) 



appearance and arrangement vary, but it is described by 

 Masterman thus: " It consists of a more or less complex 

 double layer of cells of which the outer layer lines the 

 epiblast and the inner covers the hypoblast. These two 

 layers enclose a spacious cavity, called the ccelum, which 

 is usually filled with a nutrient fluid. The ccelum is not 

 usually continuous, but it may be divided in the median 

 plane by dorsal and ventral mesenteries, which are 

 double and serve to support the hypoblastic canal; or 

 it may be divided up by .lateral mesenteries or septa 

 running transversely to the long axis of the organism. 

 The mesoblastic walls later form the muscles, skeletal 



