V THE DEVELOPMENT OF THE FROG 95 



in origin, as it certainly is in some of the Amphibia and in 

 many other vertebrates, or whether', as maintained by Mor- 

 gan, it is developed from the mesoderm, is a matter about 

 which there is a difference of opinion. Miss H. D. King ^ 

 has recently studied the formation of the notochord in Biifo 

 lentiginosiis and Rana paliistris, and has come to the con- 

 clusion that the notochord in the anterior end of the 

 embryo arises from the mesoderm, whereas in the posterior 

 part of the embryo it is developed from both mesoderm and 

 entoderm. 



External Changes. — At the time when the blastopore 

 is nearly closed the egg is still in a spherical form, except 

 that along what is to be the dorsal side of the body of the 

 embryo there is the beginning of a broad depression known 

 as the primitive groove. On either side of this are two 

 folds, the inner and the outer medullary folds, which are 

 continued as an elevation around the anterior end of the 

 primitive groove and are produced backward on either side 

 of the blastopore. The outer medullary folds gradually 

 fade away, but the inner ones become elevated and arch 

 over the groove between them. Finally the two inner folds 

 meet and fuse along the median line, converting the groove 

 into a tube. The point where they first fuse corresponds 

 to the neck region of the embryo ,; and the closure of the tube 

 proceeds both forward and backward from this point. The 

 fusion extends backward so that folds on either side of the 

 blastopore close in above that opening in such a way that it 

 becomes no longer visible from the outside. As the medul- 

 lary tube is completed it is constricted off from the ecto- 

 derm above, and the latter becomes continuous over the 

 mid-dorsal line. Subsequently it develops into the brain 

 and spinal cord of the embryo. 



'^ King, Biol. Bull., Vol. 4, 1903. 



7 



