420 THE ORIGIN OF VERTEBRATES 



of its internal tissue into a tissue of a very definite character, 

 but also by the cilia in its canals, and the steady increase in the 

 number of its funnels. It appears, therefore, to be the conversion of 

 an excretory organ into an organ for the transference of fluid out of 

 the coelom into a special tissue, i.e. into its blood-sinus ; in other 

 words, into an organ which must be classed as belonging to the 

 lymph-system." 



In exact correspondence with this transformation of a nephric 

 tubule into a ductless gland of the nature of a lymphatic gland, is the 

 formation of the head-kidney in the Teleostea. Thus, Weldon points 

 out that, though the observations of Balfour left it highly probable 

 that the " lymphatic " tissue described by him was really a result of 

 the transformation of part of the embryonic kidney, he did not inves- 

 tigate the details of its development. This was afterwards done by 

 Emery, with the following results : " In those Teleostea which he 

 has studied, Professor Emery finds that at an early stage the kidney 

 consists entirely of a single pronephric funnel, opening into the 

 pericardium, and connected with the segmental duct, which already 

 opens to the exterior. Behind this funnel, the segmental duct is 

 surrounded by a blastema, derived from the intermediate cell-mass, 

 which afterwards arranges itself more or less completely into a series 

 of solid cords, attaching themselves to the duct. These develop a 

 lumen, and become normal segmental tubules, but it is, if I may be 

 allowed the expression, a matter of chance how much of the blastema 

 becomes so transformed into kidney tubules, and how much is left 

 as the ' lymphatic ' tissue of Balfour, this ' lymphatic ' tissue remain- 

 ing either in the pronephros only, or in both pro- and neso-nephros." 



If we turn now to the invertebrates, we see also how close a con- 

 nection exists between lymphatic and phagocytic organs and excretory 

 organs. The chief merit for this discovery is due to Kowalewsky, 

 who, taking a hint from Heidenhain's work on the kidney, in which 

 he showed how easy it was to find out the nature of different parts of 

 the mammalian excretory organ by the injection of different sub- 

 stances, such as a solution of ammoniated carmine, or of indigo- 

 carmine, has injected into a large number of different invertebrates 

 various colouring matters, or litmus, or bacilli, and thus shown the 

 existence, not only of known excretory organs, but also of others, 

 lymphatic or lymphoid in nature, not hitherto suspected. 



In all cases he finds that a phagocytic action with respect to solid 



