36 FROM FISH TO PHILOSOPHER 



body through the tubules, while the blood cells and 

 plasma proteins were retained in the capillaries and re- 

 turned to the circulating blood. Such an elementary de- 

 vice persists today as the 'glomus' (glomus = haM) of 

 capillaries which is formed in the first, transient kidney 

 (the pronephros) in the embryos and larvae of nearly 

 all fishes, as well as in the pericardial cavity of a few 

 adult primitive forms (the hagfish, Myxine, and a few 

 bony fishes). 



Then in order to improve the drainage of the filtrate, 

 the capillary tuft was inserted in the tubule, perhaps at 

 first in association with an open coelomostome (see Fig- 

 lure 5c); ultimately, however, the tuft was pushed into 

 the blind end of the tubule (Figure sd) and the coe- 

 lomostome disappeared. This is the structure of the 

 glomerulus (diminutive of glomus) as it appears in the 

 adult kidney of all the higher vertebrates. In the em- 

 bryonic glomerulus the tubule is expanded into an en- 

 closing sphere or capsule (Bowman's capsule) which 

 serves to collect the filtrate and to direct it into the 

 tubule without transit across the body cavity. 



With the evolution of the glomerular nephron, there be- 

 gan a battle between the kidney and the reproductive 

 organs that continued for three hundred and fifty mil- 

 hon years. After the renal tubule, in taking on a pre- 

 dominantly excretory function, lost its opening into the 

 body cavity (Figure 5c), it could no longer serve as a 

 route of egress for sperm and eggs, and yet sperm and 

 eggs had to get to the exterior if the organism was to 

 reproduce. Moreover, as the body became encased in 

 armor, the multiple, external openings of the segmen- 

 tally arranged tubules had to be covered over and the 

 kidney had to make for itself a new, internal conduit by 

 which the urine could escape. This was accomplished 

 by fusing together the tubules in the posterior segments 

 on either side of the body cavity to form two ducts— the 

 'archinephric ducts'— into which the anterior tubules 



