74 DEVELOPMENT OF THE FROG'S EGG [Cn. VI 



the mid-dorsal line and the mesoderm. Further, it is said, that 

 the cord of cells in the median dorsal line remains for a longer 

 time connected with the mid-dorsal endoderm than does the 

 mesoderm at each side with the lateral endoderm, and that the 

 notochord separates from its lateral connections (right and 

 left) with the mesoderm, while it still remains for a time 

 closely fused in the mid-line with the endoderm. 



In the newt and in other urodeles the endoderm in the mid- 

 dorsal line thickens and bends upward to form a longitudinal 

 fold. The fold pinches off from the endoderm and forms a cord 

 of cells, the notochord. In the posterior end of the toad's 

 notochord the same method of development may be seen some- 

 times to take place. 1 



With such clear evidence of the method of formation of the 

 notochord from endoderm in the newt, it is not surprising that 

 embryologists have attempted to interpret the changes that 

 take place in the frog in the same way. The main difficulty 

 arises from an unwillingness on their part to derive the noto- 

 chord from the so-called middle germ-layer, or mesoderm. The 

 question therefore turns, for them, on what they will call the 

 middle layer in the frog, and what not the middle layer. 



Since, however, all the cells in this region have had a common 

 origin, the question is perhaps a trivial one ; for we cannot doubt, 

 I think, that had some of the cells in the middle line passed a 

 little to one side or the other of the median line, they would 

 have been capable of becoming mesoderm, and, vice versa, had 

 some of the lateral cells come to lie nearer to the middle line, 

 then they would have taken part in the formation of the 

 notochord. 



The notochord separates entirely from the mesoderm and 

 endoderm, and becomes rounded in cross-section. On each 

 side of the notochord the mesoderm becomes thicker, as is 

 shown in Fig. 42. The final stage in the closure of the med- 

 ullary folds and the changes that take place in the mesoderm 

 will be described in a later chapter. 



1 Field ('95). 



