282 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



the communication between the sterile or duct portion of the testis 

 and the Wolffian duct should come to be by a direct tubular prolonga- 

 tion of the vas efferens instead of by tortuous kidney tubules. That 

 such a shortening up and simplification of the channel from testis to 

 Wolffian duet does actually tend to come about in evolution is 

 demonstrated by the precisely similar series of modifications which 

 have occurred in the Anura at the front end of the urinogenital 

 network. In these not only has the series of vasa efferentia become 

 reduced to a single (anterior) member in such a form as Discoglossus 

 but the kidney tubule interpolated between it and the Wolffian duct 

 has become shortened and widened, so that there exists simply a 

 single tube leading from the testis and continued at its other end 

 into the Wolffian duct. 



Giving consideration to these various points it appears to be 

 justifiable to relate the probable evolutionary history of the sperm 

 duct of the Teleost in the following terms : — 



I. Primitively the elongated testis communicated with the 

 Wolffian duct by way of (a) a series of vasa efferentia distributed 

 along its length, and (6) the tubules of the opisthonephros. 



II. The posterior portion of the testis became sterile and functioned 

 merely as a reservoir and duct. 



III. The vasa efferentia became reduced to those connected with 

 this sterile region and finally to the hindermost one of these. 



IV. The channel formed by this together with the kidney tubules 

 into which the spermatozoa passed from it became shortened and 

 widened until it reached the condition of a simple tube leading from 

 the hind end of the testis into the hind end of the Wolffian duct. 



V. The final stage was reached by the opening of this tube into 

 the Wolffian duct becoming shifted back until its opening to the 

 exterior came to be independent. , 



Suprarenal Organs. — The organ familiar to students of the 

 Amniota and especially of the Mammalia under the name Suprarenal 

 or Adrenal is now generally recognized as being not a single organ 

 but an organic complex formed by the union of two originally 

 separate elements — the medullary substance and the cortical sub- 

 stance. These two elements arise quite independently in ontogeny, 

 the medullary substance being derived from the sympathetic ganglia, 

 while the cortical substance arises in the form of a number of thick- 

 enings of coelomic epithelium on the roof of the splanchnocoele between 

 the two kidneys. That this independence in early stages of ontogeny 

 is a repetition of a condition which occurred during phylogeny is 

 indicated by the fact that in Fishes the two elements are still 

 independent. The names medullary and cortical substance, referring 

 as they do to a topographical relation which occurs only in mammals, 

 are obviously unsuitable from the point of view of comparative 

 morphology and it is becoming customary to apply other more 

 characteristic names. The medullary substance in mammals and 

 what corresponds with it in other Vertebrates has a very characteristic 



