112 OUTLINES OF CHORD ATE DEVELOPMENT 



the frog the process of invagination is chiefly concerned in the 

 formation of the rudiments of the chorda and the mesoderm 

 of the dorsal and dorso-lateral regions, although these structures 

 are not formed wholly by invagination but also by the transport 

 of the materials of the germ ring to an axial position, and then 

 by delamination. In the development of the frog, therefore, 

 invagination is a process of minor importance. 



The actual materials out of which the axial structures of the 

 embryo are formed are to be found in the animal half of the 

 blastula, that is, in the walls and floor of the blastocoel, and these 

 in turn are distinguishable in the eight cell stage, where they are 

 contained in the upper quartet and the upper parts of the lower 

 quartet; finally they can be traced back, approximately to the 

 animal half of the egg. The later localization of the greater 

 part of these materials in a true germ ring is an important 

 characteristic in the development of the frog, and makes pos- 

 sible a close comparison of the gastrula and early embryo of 

 this form, with the conditions in the lower as well as higher 

 forms. It is also important that accompanying the down- 

 growth and closure of the germ ring there is a true confluence 

 of its lateral margins, forming a thickened axial mass of cells, 

 gradually elongating posteriorly through continued confluence. 

 In this axial region the chief organs characteristic of the frog 

 as a Chordate animal, have their origin. These organs grad- 

 ually differentiate as the mass elongates posteriorly, and their 

 rudiments, though not individually differentiated as such, are 

 thus seen to form gradually from the cells contained laterally 

 in the roots of the axial thickening, that is, in the germ ring 

 itself. This germ ring is not so clearly indicated in Amphioxus, 

 although we have seen in that form a rapidly growing ring of 

 cells around the blastopore, wider on its dorsal side, from which 

 are differentiated gradually the rudiments of the chorda and 

 mesoderm, and a considerable part of the medullary plate. In 

 Amphioxus, however, gastrulation is more nearly completed 

 before notogenesis commences, so that while there is some 

 overlapping it is not so extensive as to cause confusion of the 

 two processes. In the frog not only does notogenesis commence 



