136 WALTER HEAPE. 



Protovertebrse are formed and the axial and peripheral 

 portions of the mesoblast plates are separated from one another 

 by the intermediate cell mass. 



A cavity appears in the protovertebrse, Stage b, which still 

 exists at the close of Stage J. 



The formation of the muscle-plate commences at Stage h 

 from the outer layer of cells of the protovertebra, but during 

 Stage J, in the three anterior protovertebrse, a second row of 

 cells derived from the inner (vertebral) portion of the somite, 

 takes part in its formation, the two rows being continuous with 

 one another at their dorsal and ventral ends. 



The muscle-plates are first formed anteriorly. The outer 

 cells of the muscle-plate in Stage j are prolonged into fine pro- 

 cesses, which are connected with the overlying epiblast cells, 

 and constitute voluntary muscular fibres (fig. 51). Certain of 

 the cells of the inner layer are also differentiated into elongated 

 muscular fibres. 



I would further remark the mesoblast and epiblast cells in 

 front of the primitive streak appear always to be connected 

 together by processes. 



The Pericardial Cavity only commences to form during 

 Stage J, and is not at the close of that stage entirely separated 

 from the remainder of the body cavity. 



The Primitive Streak has the same relations in Stages e and i 

 as in Stage d, except that the medullary folds grow backwards 

 round its front end. The neurenteric canal disappears, but its 

 original position is indicated by the fusion of the germinal 

 layers at the front end of the primitive streak. 



The mesoblast of the primitive streak does not give rise to 

 the mesoblast of the body of the embryo in front of the primi- 

 tive streak, in my opinion, but extends backwards and outwards 

 and forms the wall of the allantois. The anus is formed in the 

 middle of the primitive streak. 



The Amnion is first formed at the hind end and from thence 

 extends forwards. This portion of the amnion is formed of a 

 double fold of somatopleure ; the epiblast of the outer fold 

 unites with the epithelium of the uterus. The anterior fold 



