TOPOGRAPHY OF THE EMBRYONIC DISC 



11 



fused lips of a primitive blastopore. They represent a centre of proliferation from which the 

 mesoderm is budded off from the ectoderm and spreads out to form the middle germ layer of the 

 embryonic disc. 



At the anterior end of the primitive streak this proliferation extends forward as a plate of 

 cells, the so-caUed 'head process.' The axial portion of this process is the anlage of the noto- 

 chord, the embryonic skeletal axis. It contains a canal, which opens into the primitive pit. 

 The notochordal anlage soon fuses with the underlying endoderm, and its canal forms the 

 transient neurenteric canal. 



In the mid-line anterior to the primitive streak there appears the shallow neural groove 

 (fig. 11), corresponding to a thickened plate of ectodermic cells, the neural plate. The neural 

 groove is slightly forked at its posterior extremity, in the region of the primitive node (Hensen's 

 node), which forms the dorsal lip of the primitive pit. As development proceeds, the neural 

 plate extends posteriorly, and the primitive pit is accordingly shifted backward, the correspond- 

 ing part of the primitive groove being converted into 'head process.' The primitive streak thus 

 becomes progressively shortened (cf. figs. 11 and 13). 



Fig. 12. — Topogkaphy of the Embryonic Disc. Diagram of Relations at the Length 

 OF about 1 MM. ng, neural groove, pn, primitive node, pp, primitive pit. U, upper limb. 

 L, lower limb. 



Topography of the embryonic disc. — Although only slight signs of differentia- 

 tion are visible in the embryonic disc at the stage shown in fig. 11, it is already 

 possible to map out more or less definite areas corresponding to all the various 

 regions of the future body, as shown in fig. 12. 



Beginning anteriorly, the head region is relatively enormous in size, occupying at this 

 time the entire portion in front of the primitive pit and formmg about half of the entire disc. 

 The cervical, thoracic, lumbar and sacro-coccygeal regions appear successively smaller, ap- 

 proaching the posterior end ('tail bud') of the primitive streak. It is also a striking fact 

 that the future dorsal region of the body wall, corresponding to the central portion of the disc, 

 along each side of the mid-line, is now larger than the ventro-lateral regions, which occupy a 

 relatively narrow area around the periphery of the disc. 



