30 EARLY AMPHIBIAN DEVELOPMENT 



invaginate this material, which would then differ in noway from the 

 presumptive mesoderm of the trunk. That the presumptive caudal 

 muscle material does not get invaginated is probably due simply to 

 the large amount of yolk present, which fills most of the interior 

 of the embryo and decreases the space available for material to be 

 invaginated. 



However, the activities which lead to the uprising of the neural 

 folds, and their fusion, appear of necessity to take in the whole 

 region from anterior end to blastopore, and so this presumptive 

 caudal muscle material, through the mere fact of its being left on 

 the surface, is made to participate in this essentially alien process. 

 Thus in the Amphibia the embryonic structures known as the 

 neural folds do not represent a single ultimate morphological unit, 

 but are composite and represent, in addition to epidermis, two 

 distinct sets of structures, the neural tube and the muscles of the 

 tail. The earliest stages of development of these sets of structures 

 are merely bound up in a single morphogenetic process, the forma- 

 tion of the embryonic neural folds. The distinction between processes 

 involving form-change and those involving chemical predetermina- 

 tion, which it will be necessary to discuss at more length later, is 

 here very evident. 



§6 



The formation of the gut, the notochord, the neural tube, the meso- 

 derm and coelomic cavity, and the tail, together with the elongation 

 of the whole embryo along the antero-posterior axis, are examples 

 of morphological differentiation, as a result of which the main 

 organ-systems of the embryo become roughly blocked out as re- 

 gards their position and their form. As development proceeds, the 

 remaining organs become roughed out in the same way. Owing to 

 the greater width of the groove between the neural folds in the 

 anterior region, the neural tube is at its first formation already 

 differentiated into regions of brain and spinal cord, the diameter 

 of the cavity of the tube being greater in the region of the brain. 

 From the brain the optic vesicles are pushed out on each side, and 

 become converted into the optic cups by the invagination of their 

 outer sides. Opposite the mouth of each optic cup, the lens is 

 formed as a thickening of the overlying epidermis, and eventually 



