ORGAN DEVELOPMENT IN VERTEBRATES 



265 



the definitive cloaca and anus. The other endodermal pocket starts from 

 the dorsal side of the gut and extends backwards into the tail, where it is 

 known as the tailgut. The extent to which it is developed varies greatly in 

 different species even of the same group of animals; thus in the frog it is 

 inconspicuous, while in some toads it forms a rather long tube, which 

 extends right round the posterior tip of the notochord, its lumen becom- 

 ing confluent with the cavity of the neural tube (the junction being known 

 as the neurenteric canal) (Fig. 12.6). 



Mesoderm 



Figure 12.6 



On the left, a diagrammatic longitudinal section through a newt neurula. 

 Closely lined neural plate, showing the future regions into which it will 

 develop; darkly dotted, notochord; lightly dotted, endoderm; cross-hatch- 

 ed, ventral mesoderm. On the right, section tlixough the tail region at a later 



stage. 



8. The kidneys 



There are three main types of 'kidney' developed in vertebrate embryos. 

 First, the pronephros forms in a more or less anterior position; later the 

 mesonephros appears further posteriorly; and fmally the metanephros 

 differentiates still further back. All these organs are paired, one appearing 

 on each side of the body in the so-called 'intermediate' mesoderm, i.e. 

 that lying between the somites and the lateral plate (Fig. 12.7). The 

 basic unit, from which the kidneys are constructed can be thought of as a 

 tube, one end of which opens into the coelome, while the other is con- 

 nected with a duct -which leads the secretion away towards the posterior; 

 and somewhere between these two ends, the tube becomes closely ap- 

 posed to a blood vessel. In functional kidneys, this simple unit becomes 

 highly modified and comphcated; we shall not discuss these details which 

 belong rather to the province of comparative morphology than that of 

 general embryology. But it is necessary to say something about the duct. 



