DEVELOPMENT OF THE TWO MIDDLE GERM-LAYEBS. 



121 



first the changes which may be recognised in viewing the germ-disc 

 from the surface, and to theso shall add. secondly, the more im- 



int results acquired by series of cross sections. 



At the posterior margin of the germ-disc of the Chick (fig. 81 A), 

 rhich consists of two layers lying on the yolk like a watch-glass, we 

 distinguished not only a short time before incubation, but also 

 luring the early hours of that process the crescent (*) and the 

 ?ntic groove, and had learned to recognise that this was the 

 from which the inner germ-layer arose by a process of folding 



When, during the first hours of incubation, the germ-layers grow 

 it farther on the yolk, the crescentic groove (fig. 81 B) is con- 

 ted into the primitive groove (pr). a structure of far-reaching 



icance. 



The metamorphosis, according to the excellent researches of DCVAL, 

 ikes place in the following manner : In the middle of the anterior 

 toporic lip. where the outer germ-layer bends over to become 

 mtinuous with the inner, there arises a small notch, which is 

 ted forwards (fig. 81 A *4); this gradually elongates into a 

 (fig. 81 B}. corresponding with the future longitudinal axis of 

 ' embryo, and by the following method : the right and the left halves 

 the [anterior] blastoporic lip, together with the part which bounds 

 le first notch, grow toward each other, and come in contact with 

 other in the median plane, with the same rapidity with which the 

 increases in super- 

 iezte&t. For a time, 

 i, the blastopore 

 the form of a short 

 longitudinal groove, 

 rhich. at its posterior 

 is bent around into 

 short transversely 

 crescentic horns 

 finally these also 

 ive disappeared ; they, 

 too. have grown toward 



other, toward the median plane, and have thus contributed 

 ly to the posterior elongation of the primitive groove. By this 

 rkable process of growth the whole blastopore is converted from 

 transverse fissure into a longitudinal one. 

 The accompanying diagrams (fig. 82) serve to illustrate this highly 



