ON THE FATE OF THE MUSCLE-PLATE, 105 



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 bodj^, complicate the process 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 («.), between the somatopleure and splanch- 

 nopleure. The nerve-roots alternate with, and lie at a deeper level 

 than, the muscle-plates. The muscle-plate itself consists of a double 

 layer of cells, continuous 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. Eound 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. Several of these cells occur in one 

 somite in a line from front to back. In other words, the fibres are 

 shorter than the thickness of the somite. In transverse sections the 

 fibres appear rounded with large nuclei, and are more separated 

 from one another. The cells at the upper and lower ends of the 

 muscle-plate stain most deeply. 



The bud which gives rise to the fore limb has at this date attained 

 considerable size. It projects almost directly outwards from the side 

 of the trunk (Fig. 2), growing partly from the mesoblast above the 

 angle between .somatopleure and splanchnopleure, and partly from 

 the somatopleure itself. It consists of a mass of mesoblastic cells, 

 densely packed together, especially at the surface and distal end. 



