636 EMBRYOLOGY. 



of their musculature. For the anterior and posterior limbs always 

 receive their nerves from a large number of spinal nerves. The 

 muscles are derived from the same source as the whole musculature 

 of the trunk from the primitive segments. 



It has not yet been possible to establish the derivation of the 

 musculature in Mammals and Man. For the limb-buds consist of a 

 mass of small, closely crowded cells ; it is impossible to state which 

 of these belong to the mesenchyme, which to the musculature, or 

 which to the nerves. The conditions in lower Vertebrates, on the 

 contrary, are much more favorable. 



In Selachians the fins, which correspond to the limbs of the higher 

 Vertebrates, contain, even at the time of their formation as small 

 plates, distinctly recognisable embryonic gelatinous tissue, which is 

 covered in by the epidermis. An important discovery by DOHRN has 

 established that there grow into the gelatinous tissue of the fin two 

 buds from each of a large number of primitive segments ; the buds 

 then become detached from their parent tissue and each is divided into 

 a dorsal and a ventral half the fundaments of extensor and flexor 

 musculature. Each fin therefore contains a series of muscular funda- 

 ments, which have arisen segmentally and are arranged one behind 

 another, a fact which has its weight in many other questions 

 touching the origin of the limbs. 



In Man the fundaments of the limbs take on a definite form as 

 early as the fifth week. The outgrowths have become enlarged and 

 divided into two regions, of which the distal becomes the hand, or 

 foot. In the case of the anterior extremity the front margin of the 

 hand already begins to acquire indentations, by which the first 

 fundaments of the fingers are indicated. In the sixth week the 

 three chief divisions of the limbs are recognisable, for the proximal 

 portion is now marked off by a transverse furrow either into arm 

 and fore-arm or into thigh and leg. Now, too, on the foot the toes 

 are indicated by constrictions, but less distinctly than are the fingers 

 on the hand. 



In the seventh week there are to be observed at the tips of the 

 fingers claw-like appendages, consisting of epidermal cells the 

 primitive nails. As HENSEN remarks, " The similarity of the hand 

 at this stage to the anterior extremity of a Carnivore viewed from 

 the sole is striking ; in addition to the toe-like brevity and thickness 

 of the fingers, the pads are well developed." 



With their enlargement the limbs apply themselves to the ventral 

 surface of the embryo, being directed obliquely from in front back- 



