264 PRINCIPLES OF EMBRYOLOGY 



do not agree on the position of the dividing hne between the anterior 

 part and the posterior part for which it is responsible. 



The existence of this theory has led to an intense study of the develop- 

 ment of the region in the neighbourhood of the blastopore of the neural- 

 plate stage amphibian. Bijtel (193 1) first showed that invagination is still 

 proceeding at the blastopore even after the appearance of the neural folds; 

 and further that some of the material between the posterior ends of the folds 

 (i.e. material of the neural plate itself) will actually form mesoderm (Fig. 

 9.10, p. 166). More recent authors, particularly Pasteels (1939), Nakamura 

 (1942) and Chuang (1947) fully confirm this, and demonstrate conclusive- 

 ly that the processes going on in the late blastopore are essentially invagin- 

 ation processes broadly similar to those of gastrulation proper. The main 

 differences are two. The first is relatively trivial. Throughout the earlier 

 phases of gastrulation, the dorsal midline above the blastopore is occupied 

 by presumptive notochord. hi the neurula stage, however, the last piece 

 of presumptive notochord invaginates before all the somite mesoderm 

 has moved into the interior; and in its final stages, therefore, the dorsal 

 lip of the blastopore consists of presumptive somite material. The second 

 difference is perhaps more important. Combined with the normal in- 

 rolling movement of gastrulation, there is in the late blastopore a stretch- 

 ing by wliich the tail is thrust out as an elongated structure. This 

 produces a rather complicated system of movements, but it does not alter 

 the essential fact that each region of tissue moves in a precisely defined 

 way and reaches a definite fmal position. There is no reason to believe, 

 as we are urged to do by Holmdahl, that the tail-bud is a mass of in- 

 different tissue from the general undifferentiated bulk of which the 

 posterior neural tube, somites and chorda appear. 



In the chick, the precise movements occurring in the late primitive 

 streak are not so fully known, but there seems no reason to doubt that 

 here again the posterior part of the body is produced by gastrulation pro- 

 cesses essentially similar to those which give rise to the anterior end (cf. 

 Pasteels 1939, Waddington 1952^). 



Associated with the tail-bud is the hind end of the gut. This opens to 

 the exterior through the anus, or cloaca, which is formed near the site of 

 the blastopore, but not directly out of it. In forms such as Amphibia, 

 which during gastrulation possess an open blastopore leading in to the 

 cavity of the gut, this opening is closed by the fusion of its lips as invagina- 

 tion terminates. From the hind end of the gut, two pockets are then pushed 

 out. One extends from the ventral side of the gut, and this reaches the 

 ectoderm (which may fold inwards to meet it) ; the endoderm and ecto- 

 derm first fuse, and then break down to give an opening which becomes 



