438 ZOOLOGY SECT. 



forwards into the space between the ectoderm and yolk-endoderm 

 that the foundations of the mesoderm are formed, In the anterior 

 primitive streak-region the primitive knot of the Reptiles is repre- 

 sented by a close union, for a short space, of all three layers. In 

 front of this, below the head end of the embryonic shield, as in 

 Reptiles, is a thickening of the yolk-endoderm the protochordal 

 plate, or head-process of the endoderm. This passes behind into the 

 endoderm underlying the primitive streak. 



As there is no invagination in Birds in general, there is no 

 primitive endoderm, and the definitive endoderm is formed solely 

 from the yolk-endoderm underlying the embryonic shield. The 

 notochord is formed by an axial modification of the endoderm cells 

 along the primitive knot (anterior primitive streak) region and 

 the protochordal plate. In the latter is formed the 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 blastopore in Amphioxus. Both primitive streak 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. In some 

 Birds there is an invagination at the anterior end of the primitive 

 groove, resulting in the formation of a neurenteric canal. 



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 forma- 

 tion 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, and called the pro-amnion (pr. airi). 



At an early period the vertebral plate, or dorsal portion of meso- 

 derm bounding the medullary groove, becomes segmented into 

 protovertebrse (Fig. 1075, B, pr. v.), and the lateral plate or ventral 

 portion of the same layer splits into somatic and splanchnic layers 

 with the ccelome between (Fig. 1078, B). The notochord (nek.) 

 is developed in the middle line below the medullary groove : some- 

 times it arises directly from the endoderm, as in most of the lower 

 forms, sometimes the mesoderm is formed as a continuous plate, 

 the axial portion of which is subsequently divided off as the 

 notochord. 



