170 OUTLINES OF CHORDATE DEVELOPMENT 



and receives not only the opening of the rectum, but the 

 openings of the excretory and reproductive ducts as well. The 

 urinary bladder is formed just before metamorphosis as a ven- 

 tral outgrowth from the cloaca. 



As the tail grows out, the nerve cord and notochord extend 

 into it, while the true enteron remains limited to the body 

 region, and the neurenteric canal consequently is drawn out 

 posteriorly. It soon cuts, off from the nerve cord, but for a 

 time its antero- ventral limb remains open into the rectum and 

 is known as the postanal gut. This gradually closes, and by 

 the time of hatching it is represented only by a strand of cells 

 extending posteriorly from the rectum, nearly to the tip of 

 the tail; finally it disappears entirely. Throughout the larval 

 stage the rectum remains short and only slightly dilated; 

 during metamorphosis it enlarges and elongates, forming a 

 considerable terminal portion of the alimentary canal. 



IV. THE MESODERMAL SOMITES 



All of the remaining systems are primarily associated with 

 the mesoderm. In the preceding chapter we described the 

 early history of the mesoderm and in a few words we may re- 

 call its arrangement at the time the larva is about to commence 

 its elongation. 



In the body region the mesoderm is already differentiated 

 into the thickened proximal portion along the chorda, known 

 as the segmental or vertebral plate, and the thinner peripheral 

 lateral plate, which passes around the sides of the yolk-mass 

 to the ventral surface. Dorso-laterally the lateral plate is 

 split into two sheets, the outer or somatic layer, and the inner 

 or splanchnic layer, separated by a narrow splanchnoccel or 

 rudimentary body cavity (Fig. 70, A). Through most of 

 the body region the vertebral and lateral plates are continuous 

 and the cavity of the lateral plate is continued into the verte- 

 bral plate as the myoccel At this stage, however, in the an- 

 terior body region, the vertebral plate is already transversely 

 divided into three or four pairs of somites, which have separated, 

 distally, from the lateral plate. 



