224 BIOLOGY: GENERAL AND MEDICAL 



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 



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 Koller.) 



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 

 portion, the outer layer, forming an ectoderm, comes to 

 overlie the inner layer, the entoderm, and form a kind 

 of blind sac, the slit-like opening into which is the 

 blastopore. 



If at this time the egg of the hen could be observed 

 from the external surface over the germinal disc, one 

 would see an oval area undermined posteriorly. At the 

 centre of the posterior flap a little notch soon makes its 

 appearance, which becomes deeper and ends in a groove 

 extending anteriorly half the length of the disc the 

 primitive groove in the centre of which is a linear 

 marking, the primitive streak. 



The primitive embryo thus comes to consist of two 

 germinal layers, ectoderm and entoderm, forming a 



