50 WALTER HEAPE. 



from which this section was taken is not singular in exhibiting 

 such a relation between the two. 



The Hypoblastic Mesoblast. 

 A single layer of rounded hypoblast cells similar to those 

 represented in section in fig. 30 is present throughout the 

 lighter shaded anterior portion of the area drawn in fig. 11. 

 At a somewhat later stage, however, these rounded cells in the 

 region on each side of the thickened axial hypoblast, in front of 

 the primitive streak, give rise to cells from which they are 

 themselves indistinguishable ; gradually the hypoblast situated 

 anteriorly follows suit, and eventually the whole of that 

 portion of the area in front of the primitive streak consists of a 

 plate of epiblast below which lies a mass of cells several layers 

 deep. These cells are rounded and appear throughout as do 

 the lateral masses of cells below the epiblast in fig. 33. 



Fig. 33 represents a section through the anterior region of 

 the area drawn in fig. 12, a glance at which will, I think, 

 prove the origin of these cells from the hypoblast. 



It appears to me that the continuity of the intermediate 

 layer with either of the primary layers is a safe guide as to the 

 origin of the former — by continuity, I mean such relations as 

 are shown at the node of Henson (fig. 34), where the bound- 

 aries of the three layers cannot be distinguished ; — and if this 

 be true I imagine there can be little doubt as to the origin of 

 the mass of cells above described. 



At a later period of development these cells become split 

 up laterally into two layers, a lower single layer of flat- 

 tened hypoblast and an upper layer of mesoblast several 

 rows deep. This differentiation takes place from behind 

 forwards, as does the original formation of this layer. 

 These relations are seen by comparing figs. 32 and 33 ; in 

 the former there is no trace of a separation of the cells 

 into hypoblast and mesoblast, while further back (fig. 33), 

 several cells (hy) along the lower border of the mass are more 

 flattened, their nuclei more elongated, and they stain more 

 deeply with hsematoxylin than do the remainder of the cells 



