238 ESSENTIALS OF ZOOLOGY 



also, the pore may be put into communication with the 

 enteron. The opening thus formed is at a later period con- 

 verted into the neurenteric canal. From these considerations 

 we are thus led to see that the pit, or Hensen's knot, is the 

 dorsal part of the blastopore, its front margin the dorsal lip, 

 and the primitive groove an elongated part of the blastoporal 

 involution. The further development indicates this still more 

 clearly. 



At first there are simply two layers, the ectoderm and 

 endoderm. With the appearance of the notochord and the 

 primitive streak, mesoderm is rapidly developed as a layer 

 intervening between these, and it is the development of the 

 mesoderm which marks the beginning of the activities of the 

 primitive streak. The notochord at its anterior end is formed 

 by a proliferation of the median endoderm, and as it is becoming 

 separated from the endoderm, by splitting off from it, a pair 

 of endodermal grooves is formed, which give off mesoderm, 

 also extending outwards. The mesoderm is slightly developed 

 in front but increases posteriorly to the primitive streak, and 

 thus a continuous mass of mesoderm is produced extending 

 over the whole length of the notochord and primitive streak 

 and forming two wings separating the ectoderm and endoderm. 



It is thus plain that the gastrulation takes place by the 

 delamination of the endoderm, and that the blastopore arises 

 as an infolding which rapidly becomes longitudinal. With 

 the appearance of the blastopore the gastrula changes into 

 a postgastrula. The blastopore is not marginal in birds 

 and reptiles, but is formed, as in Amphibia, as an infolding of 

 the outer layer of cells. 



The procedure thereafter is simple and direct. The ecto- 

 derm in front of the blastopore gives rise to the neural plate, 

 and it is delimited by the appearance of neural folds. The 

 endoderm below yields the notochord medianly, and it is at 

 first continuous with the mesoderm, which, as has been seen, is 

 formed besides from the lips of the blastopore that is to say, 

 the primitive groove. 



The nervous system is developed from the ectoderm in 

 front of the point marked by the anterior end of the notochord, 

 thus from the gastrula, and from the ectoderm up to the dorsal 



