2 54 J- THOS. PATTERSON. 



wenn auch unregelmassigem Rande, zeigt also ein zungenformiges 

 Aussehen. Man wird wohl nicht fehlgehen, dieses Gebilde als 

 den Anfang der unteren Keimschicht, d. h. des Entoderms zu be- 

 zeichnen. Es ist dies allerdings nicht das einzige zellige Material, 

 was innerhalb der Keimhohle zu finden ist, aber doch die bei 

 weitem grosste Menge." * (3) The third class includes those who 

 believe that the entoderm arises by a process of gastrulation, that 

 is, the upper layer turns under to give rise to the lower layer. 

 This view has been supported by Haeckel, Goette, Rauber, and 

 others. The work of Duval ('84) also has been quoted in sup- 

 port of gastrulation. This author describes the blastoderm at 

 the end of segmentation as a biconvex lens (Jentille biconvexe), 

 in which two layers can be recognized ; an upper epithelium-like 

 layer separated by a narrow fissure from a thick lower layer. 

 The deepest cells of the latter are open below to the white yolk 

 of the Nucleus of Pander. In a later stage a thickening occurs 

 on the margin where the upper layer is united with the lower. 

 Duvals calls this thickened rim the bonrrelet blastodermique. It 

 corresponds to the Randtvidst of the German authors. At the 

 posterior margin where the rim is thickest, a crescent-shaped 

 groove appears, which passes forward beneath the blastoderm as 

 a fissure separating the lower cells of the blastoderm from the 

 underlying yolk. Duval now regards the blastoderm as in the 

 gastrula stage and hence the fissure between the yolk and the 

 thick lower layer is the archenteron. It is clear that, in the 

 main, Duval's theory is one of delamination. So far as the 

 pigeon's egg is concerned the segmentation cavity is not found 

 just below the superficial layer of cells at the end of segmentation, 

 but is situated beneath the central portion of the blastoderm — 

 between the deepest cells and the yolk. 



In order to work out the history of a continuous develop- 

 mental process, such as gastrulation, it is necessary to have a 

 complete series of normal stages taken from one type. Such a 

 series is easily obtainable from the pigeon, and the following ac- 

 count of gastrulation is based upon a study of several series of 

 this birds's egg. 



In seeking for a stage at which to begin the account of gastru- 



x Loc. cit., p. 27. 



