218 G. C. PRICE, 



in the earlier stages. The comparisoo will also show that the cover- 

 ing of the glomerulus comes partly from the wall of the tubule, and 

 partly from the somatic epithelium lying between the tubule and the 

 inner angle of the coelomic pocket ; that the Bowman's capsule comes 

 from the splanchnic epithelium which forms the floor of the pocket; 

 and that the pocket itself forms the cavity of the Malpighian corpuscle. 



The pronephros (Figs. 15 and 16) extends through the space of 

 two body segments, and consists of a mass of dense mesenchyme, in 

 which are a number of tubules, not more than nine on the left side, 

 and not more than eight on the right. The exact number is not easy 

 to determine. The whole mass projects into the body-cavity. The 

 segmental duct extends into the pronephros, but it can not be traced 

 through to the anterior end. The tubules can not be traced to a 

 connection with a central duct. Two or three tubules on each side 

 open into the body-cavity , and some of the others seem to be just 

 on the point of breaking through. In Fig. 16 we have one of the 

 open tubules, pr.t^ and it will be seen to project into the body-cavity 

 something like a pronephric tubule of Petromyzon. In all the sections 

 of the pronephros the periphery is more or less broken, and no at- 

 tempt has been made to restore the missing parts. 



From what has been said , it is plain that a great change has 

 taken place in this part of the system since stage B, but how the 

 change has been ett'ected we do not know. In stage B the tubules no 

 longer opened into the body-cavity, so that the openings in stage C 

 have been secondarily acquired. Important changes must still take 

 place before the organ reaches the complicated structure of the adult 

 pronephros. 



In the development of a pronephros, as we learn from the work 

 of various investigators, there are at first independent segmental an- 

 lages, which form the pronephric tubules. These become connected, 

 with one another, the connecting part forming the segmental duct. 

 The lumen of the duct is formed by the lumina of the tubules growing 

 into the at first solid anläge of the duct. To put this in other words, 

 the union between duct and tubules is primary and not secondary, 

 and the lumen of the duct is formed in continuity with the lumina 

 of the tubules. 



In support of the above statement, it will be sufficient to refer 

 to the work of Rückert ('88) for the Selachians, and Mollier ('90) 

 for the salamander, although other investigators have found the same 

 thing to be true for other animals. 



