90 GENERAL SKETCH OF 



increase in complexity and a vascular glomerulus is developed 

 near each trumpet-shaped opening (nephrostome). A little 

 later a capsule encloses the opening of the head-kidney and the 

 glomerulus — ^shutting off both from the body-cavity. Originally 

 the waste-products passed directly from the body-cavity, but 

 they are later conveyed from the special capsules to the urinary 

 vesicle behind. As development proceeds the opening between 

 the latter and the gut closes and the fluid has a separate 

 passage posterior to the anus (ureter). In the later stages the 

 Ij'mphatic trabecular tissue in front is traversed by blood- 

 vessels and probably in some by tubules. In the last stages of 

 larval existence mesoblastic cells become aggregated along the 

 dorsal region of the segmental ducts, especially in the fore and 

 hind region, and minute sinuous tubules appear in their midst, 

 which pass down and open into the segmental ducts. In such 

 Teleosteans as the Wolf-fish, in which the later stages of 

 development are well shown, the segmental tubes are absent 

 at first in front, but develop largely when the ducts reach the 

 middle line beneath the aorta, and the pigment having also 

 greatly increased, the structure of the organs thus gradually 

 assumes the adult character. 



Gt-enerative organs. 



As soon as the segmental organs have reached their final 

 position on each side of the dorsal aorta, a strand of cells of the 

 inner fold of mesoblast (splanchnopleure) passes below them. 

 They form the generative epithelium though at first not a 

 genital ridge. On the genital ridge which afterwards appears 

 on either side of the mesentery, a groove develops, according to 

 Jungersen, which subsequently closes to form a canal. In others 

 (Physostomata), the lower edge of the genital ridge coalesces 

 with another ridge projecting from the peritoneum — to the 

 outer side. 



