STAGES G TO K. THE PROTOVERTEBR^E. 325 



seen both in transverse and longitudinal sections, and form the 

 commencing vertebral bodies (PI. u, fig. 6, and PI. 12, figs. IO 

 and 1 1 Vr}. 



At first the vertebral bodies have the same segmentation as 

 the protovertebrae from which they sprang ; that is to say, they 

 form masses of embryonic cells separated from each other by 

 narrow slits, continuous with the slits separating the protoverte- 

 brae. They have therefore at their first appearance a segmentation 

 completely different from that which they eventually acquire 

 (PI. 12, fig. ii). 



After the separation of the vertebral bodies from the proto- 

 vertebrae, the remaining parts of the protovertebrae may be 

 called muscle-plates ; since they become directly converted into 

 the whole voluntary muscular system of the trunk. At the time 

 when the cavity of the muscle-plates has become completely 

 separate from the body-cavity, the muscle-plates themselves 

 are oblong structures, with two walls enclosing the cavity just 

 mentioned, in which the original ventral dilatation is still visible. 

 The outer or somatic wall of the plates retains its previous simple 

 constitution. The splanchnic wall has however a somewhat 

 complicated structure. It is composed dorsally and ventrally of 

 a columnar epithelium, but in its middle portion of the muscle- 

 cells previously spoken of. Between these and the central cavity 

 of the plates the epithelium forming the remainder of the layer 

 commences to insert itself; so that between the first-formed 

 muscle and the cavity of the muscle-plate there appears a thin 

 layer of cells, not however continuous throughout. 



At the end of the period K the muscle-plates have extended 

 dorsally two-thirds of the way up the sides of the spinal cord, 

 and ventrally to the level of the segmental duct. Their edges 

 are not straight, but are bent into an angular form, with the 

 apex pointing forwards. Vide PI. 12, fig. 17 nip. 



Before the end of the period a number of connective-tissue 

 cells make their appearance, and extend upwards from the dorsal 

 summit of the muscle-plates around the top of the spinal cord. 

 These cells are at first rounded, but become typical branched 

 connective-tissue cells before the close of the period (PI. u, figs. 

 7 and 8). 



Between stages I and K the bodies of the vertebrae rapidly 



