1903.] 



Primitive Knot, etc., in Ornithorhynchus. 



315 



membrane, and distended by a considerable bulk of fluid material. It 

 is quite impossible to remove the shell without serious damage to the 

 delicate blastoderm. Fixation with the shell intact is imperative, and, 

 as a matter of fact, the result in the way of preservation proves quite 

 satisfactory, as evidenced by the condition of the cellular blastoderm, 

 in which mitotic figures are well preserved. But even after fixation, 

 the opening up of the ovum is attended with no little risk. The 

 inevitable evacuation of the contained fluid allows of crumpling of the 

 blastodermic membrane with possibility of injury to the embryonic 

 area. It is impossible, owing to its size and osmotic difficulties, to 

 treat the ovum throughout unopened. Even were that course practic- 

 able, the impossibility of orientation would be an insuperable 

 difficulty. 



After fixation and subsequent dehydration in graded alcohols, the 

 10 mm. egg was cleared in origanum oil and opened. The blastoderm 

 still remaining in situ in relation to the shell was examined from the 

 interior aspect, and was found to possess at one spot a small more 

 opaque area, somewhat oblong, but rather irregular. The portion of 

 the blastoderm containing this small opaque patch was photographed 

 by transmitted light at a magnification of 6*5 diameters for the purpose 

 of orientation. 



Our surmise that the area in question was of the nature of an 

 embryonic or primitive knot was afterwards confirmed by the exami- 

 nation of serial sections. 



No differentiation in way of an embryonic area in the wider sense 

 is recognisable in the photograph, nor was any such discovered in the 

 course of examination of the wall of the blastodermic vesicle in toto 

 under low magnification. We naturally concluded that the very 

 evident knot represented the earliest and only differentiated area, and 

 for a time devoted our attention solely to this area and the blasto- 

 derm in its vicinity. The portion of the blastoderm containing the 

 knot was separated from the remainder of the wall of the vesicle, and 

 was then imbedded and cut in serial sections. Examination of these 

 did in fact show that in the neighbourhood of the knot, and for some 

 distance from it, the wall of the vesicle was destitute of any indication 

 of further differentiation. But towards the periphery of the portion 

 sectioned and comparatively remote (nearly 2 mm.) from the knot, 

 itself, we found the commencement of a region of thickened ectoderm 

 with underlying mesoderm. Our attention thus being directed to 

 other manifestations of developmental activity in the blastoderm in 

 addition to the primitive knot, we found, in the portion of the vesi- 

 cular wall originally put aside, a quite extensive area showing im- 

 portant changes. These amount to no less than the establishment, 

 quite away from the region of the knot, of a distinct linear primitive 

 streak formation, surrounded by an area over which the ectodermal 



