THE MESODERMAL SOMITES 



593 



Even before the lateral plates are separated from the somites, there 

 is a tendency to segment in the lateral plate itself, close to the somite. 

 It is in this region that the excretory system is to be formed, and so 

 these portions are called nephrotomes, or the intermediate cell mass. 

 (Fig. 268.) 



This mere trace of segmentation in the lateral plate lasts a very 

 short time. In fact, the lateral plate itself never segments. 



The cavity within the lateral plates is the true coelom ; and, just 

 as in the chick, when the lateral plates extend further and further ven- 

 trally, they finally meet in the midline and fuse, thus making a single 

 coelom where there had been one on each side before. 



This ventral fusion forms the ventral mesentery, which soon disap- 

 pears except in the heart region. The paired coelomic cavities also come 

 together dorsally between the notochord and the digestive tract, and 

 fuse to become the dorsal mesentery, which remains as a suspensory 

 arrangement for the gut (Fig. 293). The gut itself later sinks more and 

 more ventrally and the mesentery is pulled ventrad with it. The blood 

 vessels supplying the digestive tract then grow downward through these 

 two layers of mesentery. 



