GERM LAYERS. 



21 



envelops the yolk mass which may be regarded as occupying a distended 

 intestine. The entoderm forms the lining of the pharynx and intestine, 

 together with their appendages which include the lungs, liver, pancreas, 

 and bladder. These develop later. The intestine acquires its anal 

 opening by the rupture of an anal plate, formed, like the oral plate, by the 

 meeting of entoderm and ectoderm. The entoderm also gives rise to the 



A B 



FIG. 20. DIAGRAMS BASED UPON RECONSTRUCTIONS OF A CHICK OF 30 HOURS. 



A, Dorsal view. B, Median sagittal section but with the entire heart, ant. n., Anterior neuropore; 

 ao., aorta; ect., ectoderm; ent., entoderm; Ht., heart; med. gr., med. tube, medullary groove and 

 tube; mes. seg., mesodermic segment; nch., notochprd; neph., nephrotome; op. v., optic vesicle; 

 p. cav., pericardial cavity; ph., pharynx; pr. St., primitive streak; rh. s., rhomboidal sinus; som. 

 mes., spl. mes., somatic and splanchnic mesoderm; v. v., vitelline vein. 



notochord, a supporting rod of cells extending from the anterior end of 

 the primitive streak, along the axial line to the head (B,nch.). It is the 

 only skeletal element in some animals. In fishes it is retained as a gelatinous 

 cord running through the bodies of the vertebrae which have formed about 

 it, and expanding in the intervertebral spaces. In man, if it remains at all 



