WOLFFIAN DUCT AND BODY IN THE CHICK. 19 



mesentery varies in thickness in different sections, but we believe 

 that the glomerulus is continued unbroken throughout the very 

 considerable region through which it extends. This point is, 

 however, difficult to make sure of, owing to the facility with 

 which the glomerulus breaks away. At the stage we are 

 describing no true Malpighian bodies are present in the part of 

 the Wolffian body on the same level with the anterior end of the 

 glomerulus, but the Wolffian body merely consists of theWolffian 

 duct. At the level of the posterior part of the glomerulus this 

 is no longer ttie case, but here a regular series of primary Mal- 

 pighian bodies is present, and the glomerulus of the head-kidney 

 may frequently be seen in the same section as a Malpighian 

 body. In most sections the two bodies appear quite discon- 

 nected, but in those sections in which the glomerulus of the 

 Malpighian body comes into view it is seen to be derived from 

 the same formation as the glomerulus of the head-kidney.'''' 



The point which is left in doubt in the above description, 

 viz. as to whether the glomerulus constitutes a continuous 

 structure, is at once decided by a study of its development. 



I may here state that it is not a continuous structure, but 

 consists of a series of external glomeruli, each of which corre- 

 sponds and is continuous with the glomeruli of the Malpighian 

 bodies found in this part of the trunk. 



The first development of the Wolffian tubules in the region 

 under consideration has already been described. They appear 

 as outgrowths from the Wolffian duct meeting outgrowths from 

 the intermediate cell mass immediately on the formation of the 

 segment in which tiiey are placed ; so that in a chick with fifteen 

 protovertebrae the Wolffian duct is connected with the inter- 

 mediate cell mass by a certain number of cell cords in the 12th, 

 13th, 14th, and 15th segments. 



The duct and cords, which have at first rather an irregular 

 outline, soon become well-defined compact structures. 



Eig. I'Z, taken from the 12th segment of an embryo with 

 twenty-two segments, represents the condition of things at this 



The Wolffian tubules in this region are derived from two 

 distinct structures — (1) the outgrowth from the Wolffian duct; 

 (2) part of the intermediate cell mass. 



The intermediate cell mass is at first continuous with the 

 peritoneal epithelium in every section ; but, as described in a 

 previous paper, this connection soon becomes lost at certain 

 points (fig. 9), and maintained at others (fig. 10). Figs. 9 and 

 10 are contiguous sections through the 15th segment of a chick 

 with twenty-two segments, showing this point. At these points, 

 where the continuity is retained, a peritoneal funnel is subse- 



