28 EARLY AMPHIBIAN DEVELOPMENT 



Structures have come to lie beneath the surface (with the exception 

 of a few sense-organs and placodes). 



Meanwhile, inside the embryo, the notochord has become an 

 elongated cylindrical rod above the roof of the gut in the mid- 

 dorsal line. A split within the substance of the mesoderm gives rise 

 to the coelomic cavity : this becomes restricted to the region of the 

 unsegmented lateral plate, and separates an outer somatic from an 

 inner splanchnic layer of coelomic epithelium. 



The formation of the tail is closely bound up with the processes 

 of gastrulation and neurulation. Although there is still uncertainty 

 concerning one or two points, the following appears to be the 

 course of events. When the neural folds arch over towards one 

 another and fuse, there is formed a double arch or vault of tissue 

 over the original dorsal surface of the blastula. The outer arch is 

 the superficial epidermis, and the inner arch is the neural tube 

 itself. A backgrowth of the hindmost part of the outer arch of the 

 neural folds gives rise to the epidermis of the tail, which of course 

 becomes progressively longer. Beneath this epidermis, and in 

 consequence of the outgrowth of the tail, the inner arch of the neural 

 folds becomes bent into a J, the bottom of the J occupying the 

 region of the tip of the tail, and is so disposed that the anterior 

 four-fifths of the neural folds, from the brain to the tip of the tail, 

 form the long arm of the J . The other arm of the J is bent ventrally 

 and forwards, and reaches from the tip of the tail to the region of 

 the blastopore; it is formed from the posterior one-fifth of the 

 neural folds. The notochord grows and stretches back between the 

 arms of the J to the tip of the tail, and that part of the inner arch 

 of the neural folds that lies dorsal to it (the anterior four-fifths) 

 gives rise to the definitive neural tube ; while that part of the inner 

 arch of the neural folds that comes to lie ventral to the notochord 

 gives rise to the myotomes or muscle-segments of the tail ^ (fig. 9). 



There is therefore no undiflFerentiated tail-bud from which the 

 structures of the tail arise: the neural tube and notochord are 

 present in the neurula, and their hind ends simply grow and stretch 

 backwards into the lengthening epidermal bag which forms the 

 tail, and the material for the muscles of the tail is also present in 

 the neurula in the hindmost part of the inner arch of the neural 



^ Bijtel and Woerdeman, 1928; Bijtel, 193 1. 



