72 The riiylogeny of the -Crura] Flexors 



plained on the ground of a separation of the profundus portion of the 

 popliteus from the superfieialis portion. 



In the opossum the upper partially separated portion of the pronator 

 tibise is very probably the equivalent of the peroneo-tibialis element, but 

 what may be the representative of the superfieialis portion of the pop- 

 liteus it is difficult to say. A possible degenerated representative of it 

 may be found in a strong tendon-like band which extends obliquely across 

 the knee joint from the external fabellar cartilage to the head of the tibia, 

 but such an identification can be at present merely a suggestion. More 

 important, perhaps, are the relations which seem to exist between the 

 plantaris and the popliteus as shown by anomalies in man, the popliteus 

 having occasionally an accessory head which often coincides with the 

 absence of the plantaris. 



If the tendon mentioned above as occurring in the opossum really 

 prove to be the representative in that animal of the superfieialis portion 

 of the popliteus, then there should be some muscular representative 

 cf it in lower forms. This may be found in that portion of the mono- 

 treme popliteus which arises from the epiphysial process of the fibula, 

 and in the reptilia it may have a representative in the plantaris super- 

 fieialis tenuis, although this seems at present very questionable. A study 

 of a greater number of forms than I have had at my disposal Avill be 

 necessary to trace out all the homologies of the popliteus, but I believe 

 that the observations here recorded make the supposition as to the com- 

 posite nature of the popliteus exceedingly probable. 



In conclusion a few words may be said with regard to the modifica- 

 tions and homologies of the plantar aponeurosis throughout the series. 

 In the urodele amphibia it is represented in the crus by the aponeurosis 

 which covers the posterior surface of the plantaris profundus III and 

 by the tendons of the plantares superficiales medialis and lateralis. It 

 receives, therefore the insertions of these three muscles, together with 

 that of the profundus II and a part of that of the profundus I, and 

 gives origin to the superficial muscles of the plantar surface of the foot. 

 With the increase in size of the lateral and medial portions of the plan- 

 taris superfieialis, a portion of the superficial layer of the aponeurosis 

 becomes separated to form the tendon of those muscles, while the rest 

 of it is covered in b}' them and remains included in the tendon of tl:e 

 plantaris profundus III-II, part of it giving insertion to the plantares 

 superficiales accessorius and tenuis. 



This is the reptilian condition, and the transition from it to that of 

 the mammalia is comparatively simple. The superficial layer of the 

 aponeurosis in the mammalia is represented by (1) the tendon or tendons 



