FOURTH PERIOD OF DEVELOPMENT. 137 



to the smaller number of muscle cells. In these parts 

 also the mesoblast does not grow forward till it 

 reaches the ventral line, but remains limited to the 

 dorsal half (Figs. 129, 131). 



"When the extension of the mesoblastic somites 

 has advanced as far as the ventral middle line, a 

 simple mesoblast lamella is seen between epiblast 

 and alimentary canal. This lamella has arisen through 

 growth of the prolongations of the mesoblastic somites 

 on both sides. It extends, however, further back into 

 those regions in which the mesoblastic somites have 

 not yet grown forward as far as the ventral line. This 

 may be observed on the side view (Fig. 60) as well as 

 in the transverse sections of the embryos (Fig. 139). 

 It is at the end of this period of development that the 

 first indications of the system of blood vessels show 

 themselves, in this mesoblast lamella in the ventral 

 line. There can be here seen in the latest stages a 

 clear canal, which may be followed from the posterior 

 end forwards. This is bounded by extremely flat 

 endothelium-like cells which form its wall. In the 

 region of the second somite, where in the ventral 

 middle line a disc-like thickening of the hypoblast 

 takes place, which forms the foundation of the first 



