264 The Nerves and Muscles of the Leg 
bud until this has become extensively developed. Afterwards this lim- 
iting membrane disappears, but there is little likelihood that cells de- 
rived from the myotomes then wander any considerable distance into 
the limb-bud. See Bardeen, oo. A capillary network connected with 
the umbilical artery and the cardinal vein is formed in the limb-bud at 
an early period. Somewhat later nerves extend into the limb. At the 
same time the mesenchyme begins to be differentiated into skeletal, mus- 
cular and dermal regions. During the development of the limb it shifts 
distally so that the distal margin of the limb-bud is brought opposite the 
27th and 28th, and sometimes also the 29th, spinal segments. As this 
occurs, bundles of nerve fibres from these more distal spinal segments 
extend into the lmb-bud to contribute to the posterior nerves of the 
limb. In the adult the most distal nerve to contribute to the nerves of 
the limb varies from the 26th to the 29th, but is most commonly the 
28th. (Bardeen and Elting, or). The number of spinal nerves contrib- 
uting to the chief nerves of the limb varies from six to nine, but is usually 
seven or eight (Op. cit.). These variations are in all probability asso- 
ciated with variation in position of the limb-bud to the spinal axis during 
embryonic development. 
The development of the main nerve trunks of the limb may be called 
the primary stage of nerve development and the associated variation in 
origin of the nerves, primary variation. As opposed to this primary 
development and primary variation we may call the growth which dis- 
tributes the nerves within the limb the secondary stage of development 
and the variation there found secondary variation. During the primary 
period the spinal nerves send fibre bundles by direct paths to certain 
cutaneous areas and muscular anlages. During the secondary period 
the cutaneous nerves extend over the surface of the limb from the areas 
to which they are first distributed and the muscle anlages become differ- 
entiated into specific muscles to each of which nerve branches are given. 
II. PRIMARY PERIOD OF NERVE DEVELOPMENT. 
The general structural relations at the period when the nerves begin 
to extend into the limb-bud are shown in Plate I, Figs. 1 and 2. In 
Fig. 1 are shown the right limb and the distal half of the trunk from 
the 17th (9th thoracic) to the 29th (4th sacral) spinal segments in Em- 
bryo II (length 7 mm., age 26 days). The limb-bud lies opposite the 
21st to the 26th spinal segments. The ccelom extends to a point opposite 
the 26th segment, but in the region of the limb it does not extend so far 
dorsally as in the thoracic region. In the figure several of the myotomes 
