WOLFFIAN DUCT AND BODY IN THE CHICK. 45 



sively enclosed in a special part of the body cavity, giving rise 

 to the commencement of the secondary glomeruli. With this 

 division of the glomerulus segmentally, and of each segment of 

 it into further secondary glomeruli, each lying in a specialised 

 part of the body cavity, the openings of the segmental duct 

 began to fold and divide, incompletely at first, into special open- 

 ings, one for each secondary glomerulus. T'inally, this division 

 was completed, and the segmental duct communicated by a 

 number of openings in each segment with specialised parts of 

 the body cavity containing a portion of the original aortic ridge. 

 The specialised parts containing these glomeruli being still open 

 to the body cavity, and the glomeruli being still all distinctly 

 attached by a common stalk to the walls of the body cavity, and 

 the intermediate parts of the original continuous ridge having 

 completely vanished, now the capsules enclosing the glomeruli 

 became more and more completely marked off from the body 

 cavity. The openings putting them in communication with the 

 segmental duct elongated into tubules which became coiled, and 

 the glomeruli themselves gained a greater independence of each 

 other by a development of intermediate tissue. 



A trace of the original state of things has descended to the 

 present time in the pronephros, with its continuous glomerulus 

 opposite the opening of the segmental duct, and placed in a 

 specialised part of the body cavity. Differences in structure 

 from the supposed primitive state of things have of course arisen, 

 in consequence of the specialisation of the pronephros as the 

 larval excretory organ. 



In the same way a trace of the division of the primary 

 glomeruli into primary, secondary, &c., glomeruli, is left 

 in the curious development of the external glomeruli of the 

 anterior part of the Avian mesonephros. Only in this case no 

 cause can apparently be given for the retention of this primitive 

 feature of development. 



An examination of an early stage in the development of the 

 Avian Wolffian tubules, when the primary and secondary 

 tubules are both fairly well established, but not very compli- 

 cated in structure, points very distinctly to the fact that the 

 glomeruli of the two tubules are parts of one primitive glome- 

 rulus. They appear to be continuous, and while one looks 

 ventrally, i. e. the so-called primary glomerulus, the other looks 

 dorsally. A glance at the accompanying woodcut will make 

 this clear. 



If this drawing of a section through the Wolffian body of a 

 chick in a part with primary and secondary tubules, be compared 

 with fig. 24, which is from the anterior part of the same chick 

 where there are no secondary tubules, it will be seen that the 



