THE vertebrates: the amphibia and birds 167 



plate only the first dozen or so somites have been invaginated, and the 

 mesoderm of the greater part of the trunk, and the whole of the tail, is 

 formed from the small area which still remains on the surface (p. 264). The 

 formation of these organs involves a great increase in length, and the 

 tendency to axial elongation in the midline takes on a new lease of life 

 in this slit-blastopore region. The focus of this elongation is, according 

 to Pasteels, not quite at the blastopore itself, but slightly anterior to it, 

 at the most posterior limit of the invaginated notochord. 



{b) Birds 



Although the gastrulation movements in birds are more complex than 

 in Amphibia, they are perhaps easier to visualise and to describe, since 

 they occur not in a sphere, but in a flat circular disc. The main comphca- 

 tion which is introduced is a double streaming movement along the mid- 

 line, forwards at an early stage, and backwards later on. There is moreover 

 another important difference from the Amphibia in connection with the 

 type of growth which is proceeding. In the latter group the total mass of 

 the egg is more or less constant during gastrulation; 'growth' consists in 

 the divison of cells into smaller units, and the conversion into living sub- 

 stance of yolk, already contained in the cells. In the birds, on the other 

 hand, the greater part of the yolk lies outside the tissues, and during gastru- 

 lation this is being digested and assimilated, so that growth involves an 

 actual increase in the cellular mass. 



We have seen (p. 155) that at the beginning of gastrulation in the chick a 

 lower layer of cells is already present, formed either by delamination or 

 by migration from a posterior region of the blastoderm. This remains 

 more or less in situ as the endoderm. It undergoes a general spreading out 

 to cover the whole underside of the blastoderm, which may involve 

 some posterior-to-anterior streaming, but if this occurs at all it is difficult 

 to investigate by vital staining methods, and little is known about it. 



The upper layer of the blastoderm consists of the presumptive meso- 

 derm and ectoderm. (For which reason it is sometimes referred to as the 

 mesectoderm, but since this word has been used in other senses, it is 

 better to call it the epiblast.) The formation of mesoderm begins at an 

 early stage. The site of its formation is the primitive streak, the linear 

 thickened area whose appearance in the posterior part of the blastoderm 

 we have taken as the signal for the beginning of gastrulation. At the 

 primitive streak there is no open channel leading from the outside to- 

 wards the interior, such as one fmds at the amphibian blastopore. Never- 

 theless, vital markings show that tissue which originally lay on the outer 

 surface of the epiblast streams from both sides towards the streak, turns 



