The pronephros of Chryserays marginata. 39 



The relations of the pronephric duct to ectoderm and endoderm 

 are discussed later. 



Before going- into details about the pronephros, it is necessary 

 to recall the fact that the determination of the exact body-segment 

 in which any portion of the kidney is situated becomes increas- 

 ingly difficult as we pass to the later embryos M and N and all 

 subsequent stages, on account of the fact that the embryos are no 

 longer straight, but have acquired a curved form. Hence, if in 

 sectioning, the plane of the sections in the front region of the 

 trunk is perfectly transverse, the sections further back will be 

 oblique and vice versa. So that to get a continuous series of trans- 

 verse sections the individual sections ought to be wedge-shaped. 



Furthermore — at the front end of the body the anterior proto- 

 vertebrae (immediately behind the ear) show signs of degeneration. 

 It seems unlikely however from an examination of the tables pre- 

 ceding the description of the kidney in these embrj^os that more 

 than one protovertebra becomes appreciably reduced in size, although 

 in embryo N the second protovertebra on each side also shows signs 

 of degeneration. 



In embryos I — M there arises in segment VI a vascular out- 

 growth which projects into the coelom and which has every appea- 

 rance of an external glomerulus or "glomus". This vascular projection 

 can be traced backwards as a continuous structure into segment VII 

 and beyond, although its appearance and relations to the coelom 

 become modified along with the modifications which affect the 

 pronephros in its posterior region. The beginning of this vascular 

 outgrowth is so constantly associated with segment VI, that I have 

 been led to regard its position opposite the seventh apparent segment 

 of embryo N as due to oblique sectioning and not to any alteration 

 in position, viz. backward shifting of the vascular structure, although 

 it may be that this embryo has reached the stage when the 

 pronephric traces in segment VI are disappearing. With regard 

 now to the extent of the pronephros, the tubules of this organ are 

 found (in embryo I) with certainty in segments VII, VIII and IX, 

 and imperfectly differentiated in X; in embryos J, K and L in 

 segments VII, VIII, IX and X; and in the case of embryos M 

 and N in segments VII, VIII, IX, X and XI, although the anterior 

 tubules are becoming degenerate. Again, since the tubules in 

 segments X and XI may open into the nephrotome (instead of to 

 the coelom direct) and since the nephrotome tends to become closed 



