648 



The younger of the two embryos is the one of Torpedo ocellata. 

 In it the duct is not yet formed. The pronephros consists of a long 

 involution of the somatopleure , its hinder end has attached itself 

 to the epiblast, but as yet there no traces of a segmental duct, nor of 

 the proliferation of epiblastic cells to form it. 



The Scyllium embryo ' ) is fortunately more advanced, but young 

 enough to show very clearly the epiblastic origin of the duct. It is 

 the only young Scyllium embryo in my possession , and was pre- 

 served in osmic and chromic acids by my former teacher Prof. Milnes 

 Marshall, to whom I owe the gift of it. The embryo was naturally 

 very brittle, but the epiblast and mesoblast are splendidly preserved. 

 The cell outlines, and the developing segmental duct, are as clear 

 as the figures given by Flemming ^ ) in the rabbit. 



It would merely be repeating Flemming's figures with very slight 

 variations, if I were to illustrate the formation of this duct in Scyllium. 



Nor need I say more than that I can fully confirm van Wlthe^) 

 in his statements 1) of the mesoblastic origin of the pronephros, 

 2) and of epiblastic nature of the duct, and the distance with which 

 it is fused with the epiblast. 



It is attached to the epiblast in several sections, and disappears 

 gradually, the last traces of it being a faint bulging inwards of the 

 epiblast. A growth backwards of special cells, such as occurs in the 

 formation of the lateral line, is here quite out of question. 



With van Wijhe's speculative conclusions I cannot agree, though 

 I need not say I fully recognise the great value of his contributions 

 to Vertebrate morphology. 



As VAN WijHE remarks "die Phylogenie des Systems scheint 

 nun klarer", but it is unfortunately only clearer in the sense that the 

 facts are getting out of the region of doubt, for the meaning 

 of the facts is undoubtedly about to lead to diflerences of opinion. 



body somites, there is for some distance along the epiblast, in the region 

 in which in the rabbit the duct is formed a thickening of epiblast. 

 I believe the development from this thickening to take place what I may 

 term the loose proliferation of epiblastic cells towards the mesoblast, i.e. 

 towards the somatopleure. The proliferation I believe to be somewhat 

 precocious, and it is loose in the sense that its cells do not form a rod, 

 but are comparatively few in number. This much is certain ; the duct on 

 its first formation lies outside the mesoblast, as Gassee showed, and is 

 certainly not formed from the somatopleure. 



1) Probably Scyllium canicula, 



2) Flemming, op. cit. Taf, XI, Fig. 6 c — f. 



3) VAN WiJHE, op. cit., p. 2. 



