208 HUBERT DANA GOODALE 



tained for the former. Brachet's careful work on the Axolotl 

 and frog, King's on Bufo, as well as Adler's work, all force me to 

 the conclusion, that in the more generalized forms, like Triton, 

 Rana, Bufo, etc., the mesoderm cells have arisen by differentiation 

 in situ from the roof cells of the archenteron. In more special- 

 ized forms, like Salamandra maculosa and Hypogeophis, where 

 the process has been modified by the presence of large amounts 

 of yolk, I believe that the mesoderm arises by a true invagina- 

 tion of the ectoderm cells between the endoderm and ectoderm. 



Notochord 



At the stages shown in fig. 36, or sometimes earlier, it may 

 be distinguished in the mid-dorsal line of the archenteron as a 

 mass of cells which is sometimes thicker than the lateral part of 

 the roof. They are usually more compact than the neighboring 

 lateral cells, which often are somewhat separated. It has another 

 distinguishing feature. Sometimes in sectioning, the lateral 

 parts of the roof of the archenteron are pulled away from the ecto- 

 derm but the anlage of the notochord is almost never pulled away. 

 A close study of sections show that no actual union exists between 

 it and the ectoderm. Nevertheless, they cohere in some unknown 

 way. Thus, even before morphological differences are present, 

 the anlage of the notochord may be distinguished by other char- 

 acteristics than mere position, which serve to indicate the future 

 history of these cells. 



Such characteristics serve to distinguish the forerunners of the 

 notochord cells until the stage shown in fig. 38, when it has become 

 a single layer of columnar cells. Its further development is 

 completed in a perfectly orthodox manner for Urodeles. It 

 arches up against the ectoderm, its ends separate from the meso- 

 derm and also fi'om the endoderm which closes beneath it. 

 Finally it forms a rod of cells. The time and degree of arching seem 

 to be correlated with the depth of the neural groove, for, if the 

 latter be deep, the notochord is not rounded up as much as when 

 the groove is shallow, thus lending support to Goette's views on 

 the neural groove mentioned above. 



