XIII 



PHYLUM CHORDATA 



425 



anterior or head-part of the notochord, and from it are derived also 

 the mesoderm of the head and the endodermal lining of the head- 

 part of the enteric canal. 



Immediately in front of the primitive streak the medullary 

 groove (med. gr.) appears, and the medullary folds which bound it 

 on the right and left diverge posteriorly, so as to embrace the 

 anterior end of the primitive streak, in just the same way as they 

 embrace the blastoporer in Amphioxus. In some Birds there is an 

 invagination at the anterior end of the primitive groove, resulting 

 in the formation of a neurenteric canal. Both primitive streak 



FIG. 1087. Gallus bankiva. Two stages in the development of the embryo, all. allantois; 

 am. cut edge of amniqn ; an. anus ; au. ap. auditory aperture ; au. s. auditory sac ; /. br. 

 fore-brain ; /./. fore-limb ; Ti.br. hind-brain ; h.l. hind-limb ; lit. heart ; hy. hyoid arch ; 

 m.br. mid-brain ; mn. mandlbular arch ; na. nostril ; t. tail. (After Duval.) 



and medullary groove lie at right angles to the long axis of the 

 egg, the broad end of the latter being to the embryo's right. 



The blastoderm gradually extends peripherally so as to cover 

 the yolk, and thereby becomes divisible into an embryonic portion, 

 from which the embryo is formed, and an extra-embryonic portion 

 which invests the yolk-sac, and takes no direct share in the formation 

 of the embryo. The extension of the ectoderm and endoderm 

 takes place regularly and symmetrically, but the extra-embryonic 

 mesoderm, while extending equally in the lateral and posterior 

 regions, grows forwards in the form of paired extensions, which 

 afterwards unite, so that for a time there is an area of the blastoderm 

 in front of the head of the embryo, formed of ectoderm and endoderm 

 only : this is called the pro-amnion (pr. am). 



