DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 97 



an infolding. Only by means of the theory of infolding can be ex- 

 plained the different conditions of the anterior and posterior margins 

 of the germ-disc, the more active cell-growth in the territory of the 

 crescent, the existence of a crescentic groove, and the continuity 

 of the two primary germ-layers which is demonstrable in that 

 region. Only by means of this theory, finally, is the relation of 

 Birds to the lower classes of the Vertebrates made possible. 



The discoveries which KUPFFER UND BEXECKE have made in their 

 investigations of Reptiles, which are so closely related to Birds, also 

 contribute to the elucidation of the pending controversy. In the case 

 of Lacerta agilis (fig. 55), Emys europsea, etc., there is found, as in 

 the case of the Hen at a corresponding stage of development, at the 

 boundary of the pellucid and opaque areas of the posterior end of 

 the germ-disc, an exuberant cell-growth in the form of a crescent (). 

 In the middle plane and slightly in front of this crescent there is 

 to be seen a small, transversely placed, fissure-like, opening (u), which 

 leads into a blind sac and is comparable to the crescentic groove. 

 KUPFFER rightly interprets the opening as the blastopore, which is 

 enclosed between an anterior and a posterior lip, and the cavity as 

 the ccelenteron. He also draws a comparison between the corre- 

 sponding structures in Birds and Reptiles.* 



Let us now direct our attention to the succeeding developmental 

 stages of the germ-disc of the Chick. These consist, chiefly, in 

 a constant increase of the superficial extent of the disc. 



In the freshly laid, unincubated egg (fig. 54) the outer germ-layer 

 (ak) is composed of a single sheet of closely united cylindrical cells ; 

 the inner layer (ik), on the contrary, consists of a two-layered to 

 three-layered bed of somewhat flattened elements, which are only 

 loosely associated. 



Under the influence of incubation the superficial extension of the 

 germ-disc makes rapid advances (fig. 56). In this process the outer 

 germ-layer (ak) outstrips the inner, and terminates in a region of the 



* In the interpretation of the manner in which the imagination takes place 

 in the case of the eggs of Reptiles and Birds, I differ from other investigators 

 who also maintain that a gastrulation takes place (GOETTE, HAECKEL, 

 RAUBER, BALFOUR, and others). They regard the whole margin of the germ- 

 disc as the blastopore, at which the outer germ-layer bends over to become 

 continuous with the inner layer. According to my interpretation, the invagina- 

 tion occurs at a small circumscribed place of the margin. The blastopore is 

 from the beginning surrounded by cells both on its anterior and its posterior lip. 

 The relation of the .blastopore as well as that of the germ-layers to the yolk 

 will be more fully dealt with hereafter. 



7 



