210 EIOHARD ASSHETON. 



derived from the primitive streak, at any rate for the stages 

 up to about the one drawn for fig. 6. At this time mesoblast 

 cells are separated from the hypoblast, both at points where 

 there is already existing primitive streak mesoblast, and at 

 points where up to now there has been no mesoblast, e. g. 

 under the part of the embryonal area in front of the primitive 

 streak. After this stage it is extremely difficult to determine 

 in many places what the origin of the mesoblast cells has been. 

 As, however, this involves the question of the formation of the 

 whole mesoblast, I must leave the further discussion for another 

 paper. 



I must now refer again to the figures of the embryonal area 

 (figs. 3 to 6). I have given above my explanation of the change 

 in shape, and of the process of elongation of the proliferating 

 area. 



Has the whole difference in length between the embryonal 

 areas (figs. 5 and 6) been due to the elongation of the primi- 

 tive streak itself? 



In the following paragraphs the measurements given are taken 

 from measurements made upon photographs of the specimens, 

 drawings of which are given in figs. 3, 4, 5, 6, 8, and 10. 

 The measurements, reduced to their natural magnitude, are 

 given in millimetres. 



It is evident that growth has taken place somewhere, for the 

 embryonal area in fig. 6 measured 1*72 mm., while fig. 5 

 measured but 1*38 mm. It is due hardly at all to lengthening 

 of the primitive streak area, as the primitive streak area of 

 fig. 6 measured 1'02 mm., and that of fig. 5 measured 1 mm. 

 Of course one must not suppose that the primary area of 

 growth has ceased to produce effects. Its effects hitherto 

 have been to produce a circular patch. If we suppose it still 

 to exercise a like influence the diameter of the head plate 

 will have increased anteriorly the same as it has from side 

 to side. 



Fig. 5 measures transversely '94 mm. Fig. 6 measures trans- 

 versely 1*1 mm. Therefore we may say that at least '16 mm. 

 of the length may have been due to growth of the anterior 



