The portions of the blastoderm containing the streak and its environment were removed for sectioning, double-embedded in cedar-oil-celloidin and paraffin, and eut .` in series of 10 micron sections. Their examination showed that a primitive knot was in fact présent ̃ at some distance from. what proved to be the anterior end ôf the primitive streak (cf. text-fig. 7) just as we had previously found to be the case in Spécimens D and DD. It was situated pretty close to the eut edge of the hemispherieal segment in which it lay. It had escaped détection partly, no doubt, owing to the difficulty of thoroughly investigating the margin of the hémisphère, but partly also from the fact that the cèllular tissue of the knot in this specimen was less permeated by yolk spherules, and was therefore less opaque, than was the case in other specimens of the. stage. At all events we failed to detect the knot on surface examination, and its apparent absence nonplussed us not a little, so that its later discovery in the section-séries came as a welcome relief from our perplexity. The fourth spécimen in our possession, représentative of the stage under, notice, is designated in our list as Spécimen Y. The cireur. istances of our access to this egg are somewhat interesting. Somefew years ago one of us received from the Australian Muséum a spirit-specimen labelled as the pregnant left utérus of an Ornithorhynchus shot on thé Omlgegorïg River, N.S. W., on Augusfc 11, 1884, by Mr. A. G. Hamilton, now of Mount Kembla, N.S.W.
A spirit-specimen of '20 years' standing had not appeared a promising aid to research, especially in view of the delicate character of the egg likely to be found in an only very moderately enlarged uterus. We decided, liowever, to open up the specimen by a longitudinal eut through the utérus with a sharp razor. There was then 'disclosed ^itliin thé utérine canal, and in intimate apposition with Its walls, an egg measuring 12 by 10 millims., the egg itself being divided into two soniewhat unequal hemispheroidal segments. It appeared to be in a condition of excellent préservation, and contained a quantity of loose and easily evacuated yolk-material. Both segments were then stereophotographed with the aid of the Braus-Driiner caméra. A stereograph of thé deeper of the two segments, in situ in the uterus, is reproduced in fig* 6, Plate 4. In this stereograph the primitive knot may be recognised as a small white spot in the better-lighted area of the concave segment. It was decidedly more prominent here than in Spécimen DD, formerly. described. We sought in vain for indications of the primitive streak in the vicinity of tbe knot. On the opposite segment of the egg, however, there was discovered, with some difficulty, a tolerably lëngthy portion of a primitive streak. This measured 4;9 millims. in length and ran obliquely to the eut edge of the segment which intersected it very obliquely. This caudal segment of thé primitive streak was found to end posteriorly, after widening slightly, by joining the concavity of a crescentic thickening which extended across, at right angles to the course of thé streak, for a distance of 128 millims. The apparent breadth of the 'streak itself', a short distance in front of its caudal termination, was about 0'2 millim.