EditoHal. 



XXV 



before us the history of the animated discussion as to the normal or 

 general occurrence ot the clefts is given at length. The evidence 

 seems adequate to prove that in reptiles, birds and certain mammals 

 more or fewer of the clefts actually break through. 



The specimen described was a human embryo with at least 34 

 protovertebrcC. It seems to have been quite carefully preserved and 

 every precaution was taken to preserve any traces of a membrana 

 abturatoria. The second, third, and fourth clefts were found open 

 and the ectoderm continuous with the lining membrane of the clefts. 

 It cannot fail to be of interest that even in man these clefts, whose sig- 

 nificance lies only in the common vertebrate ancestry, occasionally or 

 temporarily occur. 



The paper contains other points of interest to the morphologist 

 and is illustrated by 1 1 figures in half-tone. 



Peripheral Distribution of the Fibres of the Posterior Nerve Roots. 



Professor Sherrington^ has given an interesting account of his 

 experimental attempt to determine the segmental relations between 

 the several areas of distribution of the sensory roots. 



The field of skin belonging to each sensory spinal root he terms 

 the segmental skin field. Although in a plexus each posterior spinal 

 root gives separate contributions to many nerve trunks, the cutaneous 

 distribution of the root is composed of patches so joined that the dis- 

 tribution of the entire root forms one continuous field. Each seg- 

 mental skin field, however, spreads over adjacent ones so that each 

 region of the body is supplied by at least two dorsal roots. 



The unmodified shape of a skin field is band-like (as in the thorax 

 or neck) wrapping around the sides of the body. In the limb these 

 areas are greatly distorted and apparently dislocated. The mid-dorsal 

 line of the body may be said to be folded laterally at the origin of the 

 limbs as is also the ventral region. Upon these axes the cutaneous 

 segments are arranged. 



The edges of the foot and hand are in the segmental fields of the 

 limb about midway between the mid-dorsal and mid-ventral lines and, 

 accordingly, correspond to the lateral line of the trunk from which the 

 digits are buds. Vulva, anus, and umbilicus lie in the mid-ventral 

 line. The nipple is in the 4th thoracic field. Six segments (3d-8th 

 cervical) contribute to the anterior aspect of the fore limb ; the ist to 

 4th thoracic contribute to the posterior part. The ist-6th post -thoracic 



^ Proc. Roy. Soc. LII, 318. 



