ONTOGENESIS 223 



posite sides of the central axis. In different embryos its 

 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 crelum is not 

 usually continuous, but it may be divided in the median 



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

 fh. Cleavage-cavity; wd, white yolk; vw, lower cell layer; dw, upper cell layer of 

 the blastula. (After Duval.) 



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 

 tissues, gonads, and partly the excretory organs; and 

 the ccelum often communicates with the exterior by 

 paired canals called nephridia." 



An embryo arrived at this degree of complexity will be 

 found to conform fairly well in structure with that of 

 the unsegmented worms, though it may not otherwise 

 resemble them. 



If we now digress to see how the early development of 

 meroblastic differs from that of holoblastic eggs, we find 

 the dissimilarities chiefly accounted for by the presence 

 of the yolk. When this is large, as in the hen's egg, it 

 is impossible for blastulation and gastrulation to take 

 place in the manner described. Instead, the segmenta- 



