ON THE FATE OF THE MUSCLE-PLATE. Ill 



root. The spinal nerves are thus formed in pairs, which occupy 

 the intervals between the muscle-plates. 



Before tracing the further growth of the nerves from this 

 point, it is necessary to describe the destination of the muscle- 

 plates and the mode of formation of the limbs, as in their 

 onward development the nerves present differences according 

 as they occur in relation to the limbs, or in the intervals 

 between them. 



I. Fate of the Muscle-Plate. 



The dorsal and ventral roots of the nerves unite towards the 

 end of the third day. But even before this time the limb buds 

 have begun to appear (at sixty hours). The growth and 

 differentiation of these buds, and the relations of the muscle- 

 plates in the different regions of the body, complicate the pro- 

 cess of nerve development. 



In a Chick embryo, at the age of three days, when transverse 

 sections are made through the trunk between the limbs, the 

 muscle-plate (m. p., fig. 1) is seen as an elongated column of 

 cells lying directly beneath the epiblast, and separated from 

 the spinal cord by the spinal ganglion (sp. g.) and roots of the 

 nerve (N.). The lower end lies outside the angle (a.), between 

 the somatopleure and splanchnopleure. The nerve-roots alter- 

 nate with, and lie at a deeper level than, the muscle-plates. 

 The muscle-plate itself consists of a double layer of cells, con- 

 tinuous at the ends, and separated from each other by a very 

 evident line, the remains of the original cavity between the 

 two strata. The outer layer, whose thickness is made up of 

 several cells, consists of ovoid, spindle-shaped, or rounded cells, 

 fitting closely together, and with their long axes directed from 

 without inwards. They are sometimes multinucleated, and 

 stain deeply with carmine. Round the ends of the muscle- 

 plate they merge with the cells of the inner layer. The inner 

 layer of cells has different characters. As seen in longitudinal 

 sections, it is composed of spindle-shaped cells, which lie close 

 together with their long axes directed from before backwards. 



