WOLFFIAN DUCT AND BODY IN THE CHICK. 27 



mental duct precisely as does the pronephros in Amphibia. The 

 only difference between the two is that in Teleostei the segmental 

 duct has never more than one anterior opening, and the part of 

 the body cavity into which it opens, and in which the glomerulus 

 lies, is completely constricted off from the rest of the body cavity, 

 and comes to resemble exactly an enormous Malpighian body.^ 



I may here sum up the common features characterising the 

 ontogeny of the pronephros and its duct (segmental duct) in 

 all the forms of the Ichthyopsida in which the development is at 

 all known : 



1. The segmental duct arises first as a ridge from the 

 parietal peritoneum. This ridge usually contains a diverticulum 

 from the body cavity, and is continuously constricted off to form 

 a duct.2 



2. Except anteriorly, where the constriction only takes place 

 at intervals, leaving the openings of the pronephros (except in 

 Teleostei, where there is only one opening). 



3. These openings correspond in number with the segments 

 which the pronephros occupies.^ 



4. A vascular structure, called glomerulus, is formed, pro- 

 jecting on each side of the aorta into a specialised dilatation of 

 the anterior part of the body cavity. Myxine forms a pecuhar 

 exception to this otherwise universal fact. 



5. This dilated part of the body cavity may become partially 

 or completely separated off to form a capsule, into which the 

 glomerulus projects and the anterior end of the segmental duct 

 opens. 



6. The pronephros in all those Ichthyopsida in which it is 

 found attains a functional development, but is usually only 

 active during a period intervening between the hatching and the 

 attainment of full maturity, i. e. it only functions in the larva. 



In Elasmobranchs, which do not, so far as is known, possess 

 a pronephros, the segmental duct arises as a solid ridge from the 

 somatic layer of the intermediate cell mass in the anterior region 

 of the trunk. From this ridge there grows back a column of 

 cells to the cloaca. On the development of a lumen the seg- 

 mental duct, with its peritoneal opening, is established. The 

 duct develops quite independently of adjacent structure behind 



^ There is a functional head-kidney in adult Ganoids. It appears to 

 be formed on the Teleostean type (vide Balfour, ' Conip., Embryology, 

 vol. 2, p. 51). 



" In Petromyzon, Scott (see note, p. 445) states that this duct arises as 

 a solid rod of cells, which secondarily becomes connected with the body- 

 cavity epithelium, to form the pronephric funnels. This account, in niy 

 opinion, needs confirmation. 



3 ' Eurb.,' p. 5, p. 42. 



