126 WALTER HEAPB. 



(No. 8) it was always possible to trace processes from meso- 

 derm cells into the overlying epiblast cells. 



The elongated mesoblast cells shown in fig. 51 are more 

 extended in Stage j than they hitherto have been. 



In Stage h (figs. 27 — 29) a tendency to elongate may be 

 observed in these cells and so also in Stages g and f (fig. 17), 

 but it is not until Stage J is reached they can actually be 

 described as muscular processes. Further, this condition in 

 Stage J only exists in a marked degree in the first few anterior 

 protovertebrse ; further backwards these processes gradually 

 decrease in length. 



With regard to the inner layer of the muscle-platCj certain 

 of the cells already show a differentiation into elongated 

 muscular fibres, but they are not all of them as yet so meta- 

 morphosed. 



The protovertebrse remain at the close of Stage j still 

 separated from one another throughout their depth, and 

 between each, short blood-vessels run, which are dorso-lateral 

 branches from the dorsal aorta (fig. 52). 



The mesoblast at the front end of the embryo now extends 

 between the notochord and the floor of the neural canal (figs. 

 44j 45, and 49), the embryo having increased dorso-ventrally. 



1 find no trace of the body cavity in the head. As was 

 stated above, the splitting of the mesoblast never extends to 

 the axial portion of this part of the mesoblast^ and no cavity, 

 as far as I have been able to see, makes its appearance 

 secondarily. 



Pericardial Cavity. — The separation of the pericardial cavity 

 from the remainder of the body cavity has only commenced 

 during Stage j, and at the close of that stage the mesenteries 

 in which the ductus Cuvieri run from the body wall to the 

 sinus venosus, divide the body cavity into two dorsal sections, 

 one on each side, the pleuro-peritoneal cavities, and one median 

 ventral section, the pericardial cavity. 



These three sections are all continuous at the anterior end 

 into a single cavity surrounding the heart, which is prolonged 

 a considerable distance further forwards. 



