1903.] 



Primitive Knot, etc., in Ornithorhynchus. 



319 



in size with that now described. It is to be regretted that Semon's 

 specimen was lost through accident, so that no further comparison can 

 be instituted. There is, however, a general resemblance between 

 fig. 15 of Semon's E ti and his figs. 14 and 18 showing areas from 

 other eggs of Echidna (E 5 ) and Ornithorhynchus (0 3 ). The dimen- 

 sions of these can be calculated and are both found somewhat to 

 exceed those of our primitive knot, in spite of their relatively younger 

 ages. It is therefore difficult to say to what extent the areas referred 

 to (of E 6 and E 5 and 3 ) correspond to our primitive knot. We are, 

 however, of opinion, that Eg, though probably younger, must have 

 pretty closely corresponded, and there can be little doubt but that the 

 centre of the other areas, at least, is later on developed into an actual 

 primitive knot. The sectional figure of 3 shown in fig. 38, inter- 

 preted by Semon as showing the commencement of gastrulation, may 

 possibly bear this interpretation, but we cannot feel satisfied that the 

 granular coagulum layer between the cellular layer and the surface 

 of the yolk shown in this figure has been adequately interpreted, 

 especially in view of our complete conviction that in fig. 39 the layer 

 represented in somewhat similar fashion is beyond all doubt really 

 yolk-entoderm. 



"We do not wish to be taken as suggesting that here in 3 the 

 coagulum represents differentiated yolk-entoderm. We are simply not 

 convinced beyond all doubt that all the formed cell elements of this 

 area in 3 are disposed in the form of a superficial continuous membrane 

 as shown in fig. 38, more especially in view of the totally different 

 condition of the nearly related stage E 5 , as illustrated in the sectional 

 fig. 33, and of the condition described below in our own somewhat 

 later stage in Ornithorhynchus. 



The general form of the embryonic knot has already been noted. 

 A schematic surface projection of its outline is shown in fig. 1. This 



Fig. 1. 



figure represents a plane reconstruction of the area of the knot, com- 

 piled from the serial antero-posterior sections through the region. 

 One of the more typical of these sections is also figured semidiagra- 



