200 ON THE MOEPIIOLOGICAL CONSTITUTION OF LIMBS. 



begin to exhibit a change in their form and position. They 

 become first sessile, then pedunculated, and the peduncle then 

 indicates by an angle at its centre the formation of the central 

 joint of the shaft of the future limb — the elbow or knee-joint. 

 At the same time, what I term the plane of the limb is 

 changed. The lappet was originally developed in a plane, 

 which is coincident with the axis of the corda dorsalis. 

 This is the primary or fundamental plane of the limb ; and 

 when in this plane the lappet presents its radial or tibial 

 margin forwards towards the head, and its ulnar or fibular 

 margin backwards. When the limb leaves its primary posi- 

 tion, it lies in its secondary plane, which cuts the corda 

 dorsaKs more or less obliquely, so that the radial or tibial 

 margins of the limb are directed more or less forwards and 

 inwards, and the ulnar and fibular backwards and outwards. 

 The permanently- sessile pectoral lappets or fins of the 

 osseous fish exhibit a peculiar modification of the same 

 movement ; they rotate on a transverse axis, so that their 

 anterior or radial margins are directed downwards and their 

 ulnar margins upw^ards. In the sharks and rays the pec- 

 toral and abdominal fins continue permanently in the primary 

 plane. 



While the lappet is still in its primary plane, the rudi- 

 ments of the girdle of the future limb may be detected under 

 the integumentary covering, and therefore external to the 

 proper mass of the visceral wall of the body. In the pri- 

 mordial condition of the lappet of the wing of the chick, 

 Remak has detected four parallel streaks running to its outer 

 margin, and continuous internally with the rudimentary 

 nervous structures of the four primordial vertebrae, with which 

 the attached margin of the lappet is connected. 



Guided by embryological facts and conclusions, to the 

 more important of which I have just alluded, I have en- 

 deavoured to detect, more particularly in the osseous fislies, 



