26 ADAM SEDGWICK. 



ings from the segmental duct into the body cavity corresponds 

 with the number of segments through which the pronephros 

 extends.! 



With its excretory system in this condition the young 

 Amphibian is hatched, ^fundamentally the head-kidney retains 

 the above structure, increasing only in size until it begins to 

 atrophy, an occurrence which takes place on the development 

 of the mesonephros. 



This method of development of the segmental duct and pro- 

 nephros is fundamentally repeated in other animals which possess 

 a pronephros. 



About the marsipobranch development very little is known. 

 Tiirbringer (loc. cit.), quoting W. Miiller and his own observa- 

 tions, makes the following statements for Petromyzon : — In the 

 earliest stage which has been observed there was present at 

 about the level of the heart a groove in the parietal peritoneum, 

 which leads behind into a duct, which eventually, by a backward 

 growth, reaches the cloaca and opens into it. The anterior 

 groove or opening of the duct soon becomes divided up into 

 four openings. 



In the young Ammocsetes there is present a pronephros made 

 up of a complicated coiled duct and four or five openings into 

 the body cavity, opposite which is placed a vascular glomerulus ; 

 the whole structure extends over four or five segments.^ The pro- 

 nephros atrophies in the adult. 



In Myxine nothing is known of the development, but iu the 

 adult a pronephros has been described, which, however, is not 

 functional in old individuals (adult ?), as in them it has lost its 

 connection with the backward continuation of the segmental 

 duct. 



It^ consists of the segmental duct, which gives off dorsally a 

 number of diverticula, in which are found glomeruli, and ven- 

 trally a number of coiled canals, which open apparently into 

 the pericardial cavity. 



The fully-formed pronephros of Petromyzon then resembles 

 in structure very closely that of Amphibia, while the pronephros 

 of Myxine differs in certain important points. 



The Teleostei possess a pronephros, which persists as a large 

 organ in the adult. It develops in connection with the seg- 



* 1 'iirbringer, * Morph. Jahrbuch/ Bd. 3, p, 5. 



2 Scott, in a recent paper (' Morph. Jahrbuch,' vol. viii), states that the 

 set^mental duct iu Petroinjzon, develops as a solid cord of cells from the 

 somatic niesoblast, which subsequently becomes hollow. The peritoueal 

 openings of the head-kidney are developed as outgrowths from the anterior 

 end of this duct to the body cavity. 



3 « Jcnaische Zeitschrilt,' vol. vii, 1873. 



