138 



s. Satta, 



of both the somatic and splanchnic layers of the lateral mesoblast 

 are of irregularly quadratic forms. 



2) . The first appearance of the anläge of the pronephros is observed 

 in the region under the fourth somite ; this is the first pair of pronephric 

 tubules. The following pairs become pronounced one after the other. 

 The distal end of these tubules become confluent by the multiplication 

 of their cells and thus a direct connection is established among them. 

 These connecting pieces together from the collecting duct (Sammelrohr 

 of Ruckekt or the anterior continuation of the segmental duct) of 

 the pronephros. 



3) . When all the tubules have become developed, there are 6 pairs, 

 the first of which contains no lumen, while all the remaining pairs soon 

 acquire a tubular structure. The independent canalization of each 

 tubule proceeds backwards and into the collecting ducts, and finally 

 all the tubules and the duct on each side are set in free communication. 

 Then the ducts shift toward the median line of the body ; consequently 

 each tubule takes a latero-ventral course, and the funnels themselves be- 

 come open just ventrally. 



4) . The first pair of tubules does not develop further, and the second 

 pair degenerates after a short existence, while the 6th. pair loses its 

 connection with the duct. The disconnected tubules remain unchanged 

 for some time, but finally they disappear. 



5) . The total number of persistent tubules is, threfore, six, i. e. three 

 pairs, of which the second and third pairs are developed most vigorously. 

 This fact would explain why certain investigators believe 3 pairs to be 

 present, while we are told by some others that there exist 4 or 5 pairs. 

 In later stages, the foremost pair of the three persistent tubules is in 

 close contact with the posterior wall of the gill-chamber. 



6) . The tubules are prolonged and grow downwards, pushing their 

 way into the body-cavity, until the funnels almost meet with the cardiac 

 tube. The proximal portion of the tubules also grows enormously and 

 becomes coiled many times, so that the chest-cavity is at last filled up 

 with the convolutions of the tubules and the cardiac tube. 



