318 



Prof. J. T. Wilson and Mr. J. P. Hill. 



[Jan. 21, 



This layer of mesoderm is well represented in the stage under 

 description. But it by no means extends throughout the whole extent 

 of the blastodermic "vesicle, nor is it met with at all at or in the 

 immediate vicinity of the primitive knot. It is found most fully 

 developed in and around the primitive streak area, and here it is 

 continuous with the ectodermal thickening of the primitive streak 

 (i.e., paraxially), where it attains greater thickness and consists of 

 several layers of cells. Outwards from the line of the primitive 

 streak* it thins out gradually into a single layer, and further out 

 becomes patchy and incomplete. It is throughout distinct and in- 

 dependent of the underlying yolk-entoclerm, which can be followed 

 through the whole wall of the vesicle. We have several pre- 

 parations of small portions of the wall of the vesicle stained and 

 mounted in toto, which give clear demonstration of the huge yolk-laden 

 entoderm cells forming a continuous lining of the vesicle wall, precisely 

 as figured by Hill and Martinf in a considerably later stage. 



The ectoderm of the vesicle wall throughout the greater part of its 

 extent forms a membrane consisting of very thin flattened cells closely 

 applied to the vitelline membrane. This character it retains in the 

 vicinity and over a portion of the primitive knot. But over the 

 region already referred to as the primitive streak area, remote from 

 (posterior to) the primitive knot and extending outwards so as to be 

 practically co-extensive with the mesoderm of this area, the ectoderm 

 shows a marked change in character. Here its cells are no longer 

 flattened and squamous, but thickened and cubical, and here also 

 their developmental activity is often betrayed by the existence of 

 frequent mitotic figures. In the posterior part of the primitive streak 

 thickening, the cubical ectoderm gradually becomes more attenuated, 

 the mesoderm still continuing beneath it for some distance, even after 

 the " extra-embryonic" character of the ectoderm is assumed. 



We now come to the " primitive knot " itself, whose occurrence and 

 characters it is the special object of this brief paper to record. 



The oblong area of the blastoderm constituting the knot measured 

 0*42 mm. in one diameter (anteroposterior axis of gastrula) and 

 049 mm. in the other diameter (transverse). It is therefore compar- 

 able in general form with the area from an Echidna egg (Eg) figured 

 by Semon in his fig. 15 (Joe. at.). Unfortunately Semon gives no 

 clue to the magnification of this figure, which is on a different scale to 

 the other illustrations. One cannot therefore compare the area of E^ 



* The plane of section through the primitive streak area of the vesicle wall, 

 though its orientation was determined in rather haphazard fashion in default of 

 any visible guide, is obviously approximately transverse to the axis of the primitive 

 streak, showing the characteristic bilateral symmetry of organisation of this 

 important region. 



f Hill and Martin, " On a Platypus Embryo from the Intra-uterine Egg," 

 ' Proc. Linn. Soc, N.S.W.,' vol. 10, figs. 31—33. 



