ONTOGENESIS 205 



tissues, gonads, and partly the excretory organs; and 

 the coelum 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- 



FIG. 87. Two germ discs of hen's egg in the first hours of incubation, df, Area 

 opaca; hf, area pellucida; s, crescent; sk, crescent-knob; es, embryonic shield; 

 pr, primitive groove. (After Roller.) 



tion is partial (discoidal) and limited to a superficial 

 area where it forms a "germinal disc" or group of cells 

 which, so to speak, floats upon the upper surface of the 

 egg. This germinal disc after developing to a certain 

 point undergoes differentiation into a superficial layer 

 and deeper layers which are separated by a narrow 

 interval or space, which constitutes the cleavage cavity. 

 Thus, through a modification necessitated by circum- 

 stances, the homologue of the blastula is produced. As 

 the blastula is perfected, gastrulation takes place not 

 by the simple invagination of one side of the sphere, 

 for the segmentation cavity is not spherical, but through 

 a combination of folding and invagination by which one 



