126 



VERTEBRATA. 



like organs the so-called vascular glands, spleen are inserted into 

 the course of the lymphatic vessels. 



Urinary organs or kidneys are generally present. They have the 

 form of paired glands and lie beneath the vertebral column. The 

 first rudiments of the kidneys appear in the form of organs resem- 



** 



Ilk 



FIG. 580. Diagrammatic longitudinal sectioii through an ideal vertebrate embryo (after 

 Balfour). a, After the completion of segmentation, b, Later stage in which the 

 mesenteron is being formed at the hind end of the embryo (gastrula) . c, Stage in which 

 the neural canal is closed and communicates with the alimentary canal. EC, ectoderm ; 

 Ent, entoderm ; Ms, mesoderm ; Ffi, segmentation cavity; DA, alimentary cavity; Nr t 

 neural canal ; Ch, notochord. 



bling the segmental organs of Annelids. Peritoneal invaginations 

 (urinary tubules), which communicate with the body cavity by funnel- 

 shaped openings, come into connection with the primitive kidney-duct 

 (archinephric duct) which is the first part of the system to appear 

 (ccmpare vol. i., p. 76, fig. 71). The ducts of the kidneys the ureters 



