ORGAN DEVELOPMENT IN VERTEBRATES 261 



produces a tubular primitive gut reaching from the blastopore, which 

 occupies the position of the anus, to the head. It was for long beheved that 

 the cavity of this remained as the lumen of the adult gut, and that the 

 varioiis organs associated with the gut developed as outpocketmgs from 

 Its walls. The most strikmg of these pockets develops just posteriorly to 

 the pharynx, and is usually considered to be the rudiment of the liver 

 Accordmg to recent studies by Balmsky (1947), however, this convention- 

 al mterpretation is in error. He claims to have shown, by vital staining 



Figure 12.5 

 Semi-diagrammatic longitudinal sections of hair follicles in the mouse. On 

 mk RNr^r '" K " -Ipl^ydryl groups, on the right to show cytoplas- 

 mic RNA. S.G., sebaceous gland; D.P., dermal papilla. (From Hardy 1952 ) 



experiments, that the so-called liver pocket continues to elongate bac 

 wards until it eventually reaches right back to the blastopore region In 

 fact It constitutes the whole of the defmitive gut, and outfoldings which 

 wi give rise to the liver, the pancreas and other organs appear in its 

 walls, not in those of the primitive gut, which eventuaUy disappears 

 completely. These conclusions relate to Urodela. Nakamura and Tahara 



icture '"^ ^'''''^ ^^^ ''^''^^'°'' '' """""^ ^"^^ ^^^ conventional 



In the chick the rnain stretch of intestine (the midgut) is formed by the 



contmued backward extension of the pocket of the foregut. A similar 



