PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 633 



different stages cannot be compared without taking into account the individual variation 

 in the position of the cloaca. In comparing different stages, it is most convenient to 

 transmute all the values into the corresponding values for a hypothetical specimen of 

 that stage in which the cloaca is in some standard position, say under M56, and the 

 pelvic fin two myotomes in front of this (which are the approximate averages for 

 Lepidosiren). Proceeding thus, we see that in the youngest embryo described the 

 ectodermal thickening may be taken as opposite M54, since in the actual specimen it 

 was at M56, the cloaca being at M58. At stage 31 + (second stage described) it may 

 be taken as opposite M51 (middle) to M54 (hind end). In stage 38 the width of the 

 attachment of the limb is much less — about the length of one myotome, and this one 

 M54. I interpret this as follows : — The long stretch of modified ectoderm in stage 

 31 + , of which only the hinder end, represents the rudiment of the future limb, marks 

 the backward track, or at least the hinder part of it, of the fin. This hinder part is 

 the first to appear, in accordance with the well-known embryological rule that the 

 rudiments of more complex structures tend to appear earlier than less important organs 

 of equal, or even greater, phylogenetic age. ( Cf. the very precocious development of 

 the eye.) 



In fact, we may say that the evidence supplied by Lepidosiren points to the con- 

 clusion that the pelvic fin has migrated from a more forward position as far backward 

 as the obstruction of the cloaca allowed. 



I should like to draw attention here to certain phenomena of developmental 

 mechanics which might have resulted in a tendency to backward migration into the 

 trunk of the posterior visceral arches. 



Firstly, the topographical relations of the ectodermal rudiments to the ventral 

 processes of the myotomes which take part in muscularising the fins, i.e. the conver- 

 gence of a long stretch of the latter towards a comparatively small area of the former, 

 points to the possibility that this ectodermal rudiment acts as the incentive for the 

 myotomes in its neighbourhood to send out mesoderm cells to it. Analogous cases of 

 the dependence of the development of one part of a structure upon the presence of 

 another, earlier formed, part of it are known ; for example, the development of a lens 

 at the spot where the optic vesicle touches the skin in larvae of Rana palustris and R. 

 sijlvaticse, even when the vesicle has been cut out from its original position and dis- 

 placed so as to be underneath quite another area of the skin ; and, conversely, the failure 

 of the lens to develop in the absence of the stimulus of contact between optic vesicle 

 and skin (Lewis). 



Secondly, we have the experiment of Harrison, of grafting the tail of Rana 

 palustris on the body of R. virescens tadpole, in place of its own amputated tail, and 

 vice versa. In these cases, for a long time the tissues of the two components are sharply 

 marked off from one another by their specific coloration. He finds that the ectoderm 



