i GASTBULATION 51 



meeting to form a closed ellipse. The yolk-plug differs from that of 

 amphibians merely in its being elliptical in outline instead of 

 circular. 



During these changes the anterior or dorsal part of the gastrular 

 lip grows actively backwards over the surface of the yolk-plug, the 

 portion of yolk-plug which is covered over in this way becoming 

 added to the floor of the archenteron and its superficial cells 

 becoming converted into archenteric endoderm. 



The last, phase in the closing of the gastrular opening consists 

 in its lateral walls approaching the mesial plane so that the opening 

 assumes the form of a longitudinal slit (Fig. 29, D). Part of this 

 slit persists for some time as a neurenteric canal — a communication 

 between the enteric cavity and the cavity of the neural groove or 

 tube (Fig. 32, C) — while a portion farther back seems to be repre- 

 sented by the anus although in this case the patent opening 

 disappears temporarily so that no absolute continuity can be traced. 

 In the region where the lips have undergone fusion there persists 

 for a time complete continuity between the different cell- layers. 

 The study of sections shows this continuity to be precisely the same 

 as that which occurs in the primitive streak of Birds and Mammals 

 (Fig. 32, B, D, E), and we have thus suggested a clue to the meaning 

 of that otherwise enigmatical structure. 



It will have been gathered that the archenteric cavity has 

 become greatly reduced in importance in the Beptile as compared 

 with the more primitive vertebrates. It has become much reduced 

 in relative size, 1 and it soon loses its individuality, becoming merged 

 with the irregular segmentation spaces lying beneath the blastoderm. 

 Correlated with this we can no longer speak of direct conversion of 

 the archenteric cavity into the enteron or alimentary canal, except 

 to a trifling extent. The latter arises, for the most part, as will he 

 shown later, in a quite different manner from the secondary 

 endoderm. 



Birds. — In the Beptile, as compared with one of the more 

 primitive anamnia, the main peculiarity of the gastrulation process 

 lies in the fact that the cavity which opens to the exterior by 

 the blastopore is normally of double origin, only its posterior 

 portion being derived from archenteron. Consequently the layer 

 of endoderm which lines it is only to a comparatively small extent 

 derived from the archenteric lining, the much greater anterior part 

 being formed from elements of independent origin. 



In the Amniota above Keptiles the replacement of archenteric 

 by secondary endoderm has gone still further, inasmuch as the 

 formation of an archenteron has in them either completely dis- 

 appeared from development, or at the least is reduced to a faint 

 vestige, and the endoderm is therefore entirely secondary. 



1 In some forms, such as Lacerta, the archenteric portion of the enteron appears to 

 be for a time much shorter relatively than in others (e.g. Platydactylus) hut this is 

 corrected later on by active overgrowth on the part of the dorsal lip (Will, 1895). 



