THE EMBRYO. 



285 



or notockord. At the circumference of the embryo and 

 beyond it, the germinal membrane splits up into two layers, 

 and this division proceeds completely round the yelk; but if 

 a section be made through the embryo, three layers, much 

 more closely connected, are seen; the innermost of which is 

 converted into the epithelial lining of the alimentary canal 

 and its appendages, while the middle layer forms the prin- 

 cipal part of the body, and the outer layer, so far as it lies 

 within the primitive groove, is devoted to the formation of 

 the cerebro-spinal axis, and, beyond the margin of the groove, 

 is converted into the cuticle of the whole body. 



Fig. 148. PBIMITIVE GROOVE OF Fig. 149. SECTION OF MAMMA- 

 RABBIT, magnified five dia- MAN OVUM: diagram, a, Outer 

 meters, a, Area ppaca ; b, area layer of germinal membrane ; 

 pellucida; c, primitive trace, b, inner layer; c, primitive 

 with the groove in the middle. groove, and, beneath it, section 

 After Bischoff. of chorda dorsalis, with the rest 



of the middle layer on each 

 side. ' 



208. At this point it may be well to pause for a moment, 

 and direct the student's attention to some of the peculiarities 

 of the eggs of birds, since it is in the hen's egg that by far the 

 easiest opportunity is obtained for studying embryology from 

 nature. The hen's egg becomes impregnated in the upper 

 part of the oviduct, and the cleavage of the yolk is confined 

 to a white spot at one side called the cicatricula. This 

 takes place by first one cleft appearing, then four, then 

 others between them, radiating from a centre, and portions 

 between these radiations being separated irregularly, and 

 afterwards subdividing; but the yolk beyond the cicatricula 



