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WOLFFIAN DUCT AND BODY IN THE CHICK, 4.45 
ings from the segmental duct into the body cavity corresponds 
with the number of segments through which the pronephros 
extends.1 
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. 
Fiirbringer (loc. cit.), quoting W. Miller 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 Ammoceetes 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 in 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 proenephros 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 Fiirbringer, ‘ Morph. Jahrbuch,’ Bd. 3, p. 5. 
2 Scott, in a recent paper (‘ Morph. Jahrbuch,’ vol. viii), states that the - 
segmental duct in Petromyzon, develops as a solid cord of cells from the 
somatic mesoblast, which subsequently becomes hollow. The peritoneal 
openings of the head-kidney are developed as outgrowths from the anterior 
end of this duct to the body cavity. 
3 * Jenaische Zeitschrift,’ vol. vii, 1873. 
