1903.] 



Primitive Knot, etc., in Ornithorhynclius. 



317 



the larger diameter of the embryonic knot, and parallel to its pre- 

 sumable axis, on the supposition that a clear spot near one of its 

 margins represented the position of the gastrula opening or blasto- 

 pore, thus determining the true posterior margin of the knot. 



Examination of the series did ultimately show that the sections 

 were approximately longitudinal sections through the gastrula cavity. 



The general character of the egg at the stage under consideration 

 must be briefly referred to. At the period now dealt with, the 

 formerly yolk-laden ovum has become transformed into a large 

 blastodermic vesicle with fluid contents, amongst which are large 

 numbers of dispersed yolk-spheres of the original yolk-mass. One 

 can, without hesitation, homologise the interior of the vesicle with 

 the subgerminal cavity of a Sauropsidan egg, extended so as to 

 include by liquefaction the whole of the yolk itself. Ornithorhynclius 

 indeed may be said to afford an actual demonstration of the trans- 

 formation of a Sauropsidan subgerminal cavity, such as is figured 

 in its first beginnings in Semon's figures 36 and 38 of his " 3 ," 

 into the cavity of a mammalian blastodermic vesicle, thus supporting 

 Keibel's view of the correspondence of these cavities. 



The establishment of the vesicular stage has been effected by the 

 extension of the cellular blastoderm completely around the yolk, a 

 condition representing a considerable advance on that existing in 

 Semon's figures " E 5 " and " 3 ." He does not record the condition in 

 this respect of his " E 6 ." He was also unable fully to determine the con- 

 dition of " E 7 " in this respect. We believe that the latter must have 

 been very similar to our present stage, judging from the figure he gives 

 of a portion of the extra-embryonic blastoderm. 



Not only is the cellular wall of the blastodermic vesicle complete in our 

 specimen, but it is already bilaminar throughout, and trilaminar over a 

 not inconsiderable area. A complete layer of yolk-entoderm (" second- 

 ary " entoderm) has been differentiated lining the cavity, except under 

 the small area of the primitive knot, with whose tissue it is continuous. 

 The yolk-entoderm cells are large and swollen, being distended by 

 yolk-spheres of various sizes and somewhat different staining capacities. 

 Owing to their yolk-laden character their protoplasmic contents are 

 relatively greatly reduced. The nuclei are, as a rule, large and vesi- 

 cular. In the least successful sections the entoderm cells may be 

 broken up and more difficult of recognition. We are firmly convinced 

 that it is owing to the imperfection of the section shown in Semon's 

 figure 39 from his "E;" that such a careful observer has been unable 

 to recognise the yolk-entoderm as such, and figures it as a zone of 

 vacuolated coagulum next the surface of the yolk. The imperfect 

 layer of angular-looking cells which he figures and regards as the 

 entoderm in "E 7 " is not really such, but forms an intermediate layer 

 •of mesodermal cells. 



