DEVELOPMENT OF THE FROG 103 



which no limbs are present and only a very indistinct head 

 is developed. Essentially, then, the frog consists of a muscular 

 body-wall forming a tube thick above, thin below, and bound- 

 ing the body-cavity. On the dorsal surface is a tube, the 

 central nervous system, imbedded in the body-wall. In the 

 body-cavity is a second tube, the alimentary canal. Around 

 the nervous tube is a bony sheath divisible into skull and 

 vertebrae. The most essential part of this skeleton is 

 a rod of cells running down the centra. In the lowest 

 vertebrates this alone persists and is called the notochord. 

 Thus reduced to its essential anatomical characters, a frog 

 consists of two tubes lying one over the other, separated 

 by a rod, the whole enclosed in a flexible muscular body- 

 wall. 



The wall of the alimentary tube and that of the body are 

 made up of three complementary layers. Counting from 

 the body-cavity inwards to the gut-cavity, the coats are 

 peritoneum, muscle, and mucous lining : from the body 

 cavity outwards they are peritoneum, muscle, and mucous 

 skin. 



The development of the frog is mainly the history of the 

 formation of these two tubes, one nervous the other digestive ; 

 of the intervening rod or notochord ; and of the coats which 

 compose the body-wall and the gut-wall. 



We may anticipate the first history. The nervous tube 

 is derived from the skin. It is folded off along the back, 

 surrounded by nerve-cells and sunk inwards. We are thus 

 left with a double tube : the alimentary canal and the body- 

 wall with its notochordal rod. 



The complementary nature of the inner and outer coats of 

 the body-cavity suggest that they have a common origin in 

 a common tissue. This would reduce the frog to a tube 

 composed of a central digestive layer surrounded by a 

 middle coat of indifferent tissue and covered with the outer 

 skin. These three coats have received names. The outer 

 one, being potentially both nervous system and skin, is called 

 by a name signifying surface-growth or epiblast ; the middle 

 one, mesoblast ; the inner, hypoblast. 



There are two ways in either of which this triple-layered 

 organism could arise from a simple beginning. Imagine a 



