156 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



genetic affinity to the gill arclies, from wliich they are separated by an 

 amazingly wide interval, as shown in Fig. 13, in which their position 

 is indicated at ff. 



STRUCTURES DEVELOPED IN THE YOUNG- SPANISH MACKEREL AFTER 



HATCHING. 



Twelve hours after the young fish has left the egg-membrane it has 

 the appearance represented in Fig. 14 ; the yelk has diminished very 

 perceptibly in size, while the anterior end of the head is considerably 

 l)rolonged forwards as compared with its condition in the recently- 

 hatched embryo shown in Fig. 13. The yelk in collapsing also leaves 

 the. hind i^ortion of the intestine in an apparently more posterior posi- 

 tion, while behind the latter the rudimentary urinary bladder al is 

 very distinctly shown as a vesicle from which the simple tubular pro- 

 longation of the Wolffian duct wd passes up behind and above the 

 intestine. The Wolffian duct appears to develop comparatively late in 

 the mackerel, as it is difficult to make out anything corresponding to it 

 before the young fish leaves the egg. It would appear to originate from 

 the peritoneal (splanchnopleural) wall of the abdominal cavity, and to 

 be continued on either side into the urinary vesicle or bladder rt?, which 

 ojiens outward immediately behind the vent. During the latter stages 

 of development, or from the fourth to the seventh day, the hind portion 

 of the Wolffian duct acquires a decided flexure just before it enters the 

 bladder, as indicated in Figs. 10 and 17. This hinder portion may be 

 considered to probably represent the rudiments of the urinary ducts or 

 ureters of still more advanced stages of development. 



"The excretory system commences [in true fishes] with the formation 

 of a [longitudinally disposed, paired] segmental duct, formed by a con- 

 striction of the parietal wall of the peritoneal cavity. The anterior end 

 remains open to the body cavity, and forms a pronephros (head kidney). 

 On the inner side of, and opposite this opening a glomerulus is devel- 

 oped, and the part of the body cavity containing both the glomerulus 

 and the opening of the pronephros becomes shut off from the remainder 

 of the body cavity, and forms a completely closed Malpighian capsule. 

 The mesonephros (Wolffian body) is late in developing."* 



I believe it possible, however, from my own studies on some forms, to 

 show that a system of segmental tubes joins the segmental ducts at a 

 late stage of development, which pass into glomeruli which lie for the 

 most part along the middle line of the dorsal wall of the abdominal 

 cavity, close to the median dorsal aortic vessel. 



Upon the origin of the generative structures of the young mackerel 

 I have made no observations, but their position in the adult would show 

 that, as in other fish-like types, they must originate as specializations 

 of tissue tracts on either side of the mesenteric suspensor of the intes- 

 tine. 



Comp. Embryol. Balfour, ii, p. 63. 



