224 BULLETIN OF THE 



plane as the third nephrostome ; and Figure G, as we have seen, shows 

 more plainly the same condition in the case of the second nephro- 

 stome at an earlier stage. 



The structure of the protovertebne in this stage (Plate V. Fig. 45) 

 merits especial consideration. Already in younger stages there is a 

 differentiation of a peripheral epithelial layer surrounding the dense cen- 

 tral mass, or kernel of the proto vertebra. Laterally this peripheral part 

 is represented by the entire somatic layer, which is separated from the 

 kernel by the protovertebral cavity (coel.). Along the median and ven- 

 tral boundaries of the somite, a layer having an epithelial character is 

 also to be seen. Thus the central mass which is to develop into the myo- 

 tome lies on the median side of the coelom, and is wholly surrounded by 

 an epithelial layer. Frontal sections show that this layer can be traced 

 inward for some distance between successive somites, both from their 

 median and lateral surfaces. Since the development of the protovertebrse 

 proceeds from before backwards, a single frontal section shows successive 

 stages in the changes which they undergo. From such a section it is 

 apparent that neither the median nor the lateral portion of the pe- 

 ripheral layer develops muscular fibres. That portion of this layer, 

 however, which is included between the kernels of successive proto- 

 vertebrse, is apparently differentiated into muscle, and becomes merged 

 in the myotomes. Very soon after the first development of muscle 

 fibres in the myotomes, the peripheral portions which have not been 

 converted into muscle separate from the central mass, and, while yet ad- 

 hering in a lamella, show evident signs of disassociation. It is to be 

 noted, that, in regions where traces of the communicating canal are still 

 distinguishable, the median peripheral kiyer, not the kernel, is seen to be 

 continuous with the splanchnopleure. The somatopleure, on the other 

 hand, may be traced, as before, into the outer layer of the protovertebra. 

 This peripheral layer I believe to be wholly converted, with the excep- 

 tion stated, into mesenchymatic tissue. In the stage before us we see 

 that it is distinctly breaking away from the myotome, and that the cells 

 are acquiring a flat tile-like form. In the following stage no layer that 

 could properly be called epithelial is present. In its stead there is a 

 considerable quantity of loose mesenchyme, and the lateral face of the 

 myotome is covered by a sheath consisting of very delicate fibrillar con- 

 nective tissue. 



Not merely is mesenchyme produced by the thin peripheral layer of 

 the protovertebra?, but in anterior regions considerable portions of the 

 kernels of the proto vertebrae also undergo a metamorphosis in this direc- 



