THE PEIMTTIYE STEEAK OF THE EABBIT. 215 



them (PC). This cavity gives rise to the pericardial cavity, 

 the inner thick wall to the muscular wall of the heart, and 

 peritoneal lining of the cavity around the heart ; the outer 

 thinner wall to the peritoneal lining of the rest of the peri- 

 cardial cavity. The cells, which seem now to be budding off 

 from the hypoblast of this spot (marked END. H.), give rise, I 

 believe, to the first of the cells which, in fig. 38, are seen to be 

 forming into a tube, which is the endothelial lining of the 

 heart [END. H.). 



Fig. 38 is from an older stage, an embryo with seven meso- 

 blastic somites, a little older than fig. 10. Although it is not 

 possible to give a decided opinion, I am inclined to think that 

 at this stage cells are still being budded off from the hypoblast 

 to form the endothelial lining of the heart. 



At the same time I must mention that at an age intermediate 

 between figs. 37 and 38 I have found a stage in which a mass 

 of cells lies between tiie PC. cells and the hypoblast cells, 

 showing no trace of a budding off either from the hypoblast or 

 the mesoblast mass marked PC 



In fig. 38 the endothelial cells may be seen to be attached 

 to both the pericardial cells and the hypoblast cells, though the 

 latter give one the impression of being concerned in the pro- 

 duction of the cells in question rather than the former. 



This band, originating as a thickening of the hypoblast at a 

 time when the embryonal area is hardly at all affected by the 

 primitive streak activity, seems to be of great use in preserving 

 the outlines of the embryo due to the primary centre of 

 growth. 



Now no part of the pericardial thickening seems to be 

 affected in the growth of the embryo caused by the secondary 

 centre of activity, the primitive streak, excepting that it is 

 absent posteriorly. Where it is present, that is, anteriorly and 

 laterally, it forms almost an accurate '' segment " of a circle, 

 the circumferential boundary of which extends through about 

 270°, or three quarters of a whole circle. 



From this I argue that all the parts of the embryo which are 

 formed within the outer peripheral boundary of the pericardial 



