THE vertebrates: the amphibia and birds 157 



marked central area pellucida surrounded by an area opaca. The embryo is 

 formed entirely within the former. The opaque area is concerned mainly 

 with digesting and liquefying the yolk, a process which is carried out 

 chiefly by the underlying cells, which in this region do not separate 

 cleanly from the upper layer, so that the whole zone remains a mass of 

 rather spongy tissue not clearly divided into an epiblast and hypoblast 

 till somewhat later. 



By the time the egg is laid, in most birds (such as the chick), the blasto- 

 derm has already differentiated into a fairly well-defined area opaca and 

 area pellucida, and, in the latter, the hypoblast and epiblast are well 

 separated from one another, except perhaps in the anterior region, where 

 the hypoblast may not yet have appeared. Very shortly after this, a 

 thickening appears in the posterior region of the area pellucida. Tliis is the 

 beginning of the primitive streak, and the first sign of gastrulation. 



2. Gastrulation: presumptive maps 



At the beginning of gastrulation, the embryos of Amphibia and birds 

 present completely different appearances. The former is a hollow sphere, 

 with a thin roof and a thick floor surrounding a large blastocoel cavity; 

 the centre of gastrulation is indicated by a blastopore, at this stage a small 

 crescent-shaped groove lying within the area of pale yolk-laden cells of the 

 blastocoel floor. The bird embryo is in the form of a blastoderm, a thin 

 sheet of cells floating over a subgerminal cavity filled with liquified yolk; 

 the blastocoel is represented only by a narrow cleft within the sheet, 

 dividing it into an epiblast above and a hypoblast below; and the centre 

 of gastrulation is indicated by the 'primitive streak', a short linear thicken- 

 ing in the posterior region of the area pellucida. The end-products of gas- 

 trulation in both forms are, however, similar in many essential respects. 

 Both contain three layers of tissue, a mesoderm lying between an outer 

 ectoderm and an inner endoderm. In both an embryonic axis is beginning 

 to develop. An axial strand of ectoderm is thickening and folding up into 

 a tube to become the rudiment of the central nervous system. Immediately 

 beneath this, mesoderm is forming a notochof tht Jong narrow rod of 

 tissue; while on either side of the notochord the ther of mesoderm is con- 

 densing into a row of separate more or less cbf val blocks, the somites. 

 And beneath the mesoderm again, the endoderdc is also foriTung an axial 

 tube, the rudiment of the gut. The movemei^a-. and foldings by which 

 these two dissimilar starting-points can be.orought to these two similar 

 end-products must necessarily differ in Jiy ways; but there is one 

 further point of resemblance which has not yet been pointed out, and which 

 renders the two forms much more easily comparable and understandable. 



