lyO PRINCIPLES OF EMBRYOLOGY 



the backward movement started at an earlier stage in the posterior region, 

 it soon acquires relatively greater speed in the neighbourhood of the 

 node, which therefore travels back down the streak, catching up, as it 

 were, the regions posterior to it. The node marks the most anterior point 

 where invagination is still proceeding, and as it moves backward along 

 the streak, the area in front of it is occupied by neural ectoderm in the 

 upper layer and already-invaginated notochord and somites in the lower. 

 Since the node is moving faster than the more posterior parts, the total 

 length of the streak is continually being reduced; but it is a long time 

 before the node finally overtakes the posterior tip of the streak and thus 

 obliterates it altogether. 



Before this happens the fundamental organs of the embryonic axis 

 appear at the anterior end much as they did in the Amphibia. The plate 

 of neural ectoderm rolls up into a groove and finally into a tube sunk 

 below the epidermis; the underlying mesoderm separates itself into a 

 median notochord flanked by thickened strips of somite material ; and 

 these become transversely segmented to form the paired cubical blocks 

 of the somites themselves. The remnant of the streak which persists in 

 the posterior all this time, where invagination is still proceeding, may be 

 compared with the slit-like blastopore which we saw remains active 

 while the neural groove is forming in the Amphibia; but in the birds 

 the structure is not only relatively larger than in the frog, but continues 

 in being to a stage in which the anterior part of the embryonic axis is 

 much further advanced. The formation of a definite gut from the endo- 

 derm will be described later (p. 252). 



4. General properties of gastrulation movements 



We have confined ourselves so far to a straightforward description of 

 the movements which carry the regions of the early gastrula into their 

 fmal positions. These are the fundamental events by which the future 

 animal acquires its organic form, and the biophysics of the process will be 

 discussed in some detail later (Chapter XX). There are, however, some 

 general points which may be mentioned here. 



The forces producing gastrulation are not entirely functions of the egg 

 as a whole, but are inherent in quite small parts of it. This emerges clearly 

 from experiments in which parts of the gastrula are isolated. For instance, 

 if in the stage with a fully formed primitive streak the chick blastoderm is 

 cut transversely into two parts, the expected backward streaming along 

 the axis takes place in both of them. This leads to the protrusion from the 

 anterior part of a 'tail' containing the axial organs (neural tube, notochord 

 and somites), while in the posterior half the medial material withdraws 



