THE PKIMITIVE STEEAK OP THE EABBIT. 213 



this area, are continually being added on posteriorly, showing 

 that the differentiation is at any rate taking place from 

 before backwards. 



(c) If this is a centre of growth there should, under the cir- 

 cumstances, be some sign of a radial disposition of tissue, which 

 there is not. 



For these reasons I think we cannot conclude that there is 

 any positive proof of a rapid addition of new cell material, 

 although an arrangement of the mesoblast cells into meso- 

 blastic somites undoubtedly takes place in situ. 



(iii) Since, therefore, we cannot find probable proof of 

 growth of new segments either anteriorly or in the area in 

 question itself, and since we do find an area immediately 

 behind the area in question and continuous with it in which 

 there is an undoubted rapid proliferation of cells (which are all 

 similar and connected, and only become epiblast, hypoblast, or 

 mesoblast according to which region they may adjoin), I think 

 it may be considered that there is very good evidence indeed 

 that the growth of the embryo of the area in question has 

 taken place by the addition of indififerent cells from this area 

 — the primitive streak. 



Although such measurements as the above are useful in 

 support of my contention, they are not satisfactory, because 

 every specimen of exactly the same stage is by no means always 

 exactly the same size. 



A very useful landmark is supplied by the band of tissue, or 

 rather an area which gives rise at first to the mesoblast form- 

 ing the pericardium, and rather later the endothelial lining of 

 the heart. This area is clearly marked as a circular band in 

 fig. 10, PC. 



In this stage the neural tube, owing to its more rapid growth, 

 has become folded forwards so as to hide the anterior part of 

 the pericardial band PC. 



In fig. 28, PI. 20, which is a median sagittal section of this 

 specimen, it is seen at the point PC. 



In fig. 9, an earlier stage, before the neural tube has become 

 thrust forward, this band is seen to follow the contour of the 



