[441] Arch. für Mic. Anat. Vol. XIV.
[442] The views here expressed about the Wolffian duct are nearly though not exactly those which one of us previously put forward (“Urinogenital Organs of Vertebrates,” &c., pp. 45-46) [This edition, pp. [164], [165]], and with which Fürbringer appears exactly to agree. Possibly Dr Fürbringer would alter his view on this point were he to accept the facts we believe ourselves to have discovered. Semper's view also differs from ours, in that he believes the Wolffian duct to correspond in its entirety with the segmental duct.
[443] “Urogenital-System d. Reptilien,” Arb. aus d. zool.-zoot. Inst. Würzburg, Vol. IV.
[444] Loc. cit.
[445] Loc. cit.
XIV. On the Early Development of the Lacertilia, together with some Observations on the Nature and Relations of the Primitive Streak[446].
(With Plate 29.)
Till quite recently no observations were recorded on the early developmental changes of the reptilian ovum. Not long ago Professors Kupffer and Benecke published a preliminary note on the early development of Lacerta agilis and Emys Europea[447]. I have myself also been able to make some observations on the embryo of Lacerta muralis. The number of my embryos has been somewhat limited, and most of those which I have had have been preserved in bichromate of potash, which has turned out a far from satisfactory hardening reagent. In spite of these difficulties I have been led on some points to very different results from those of the German investigators, and to results which are more in accordance with what we know of other Sauropsidan types. I commence with a short account of the results of Kupffer and Benecke.
Segmentation takes place exactly as in birds, and the resulting blastoderm, which is thickened at its edge, spreads rapidly over the yolk. Shortly before the yolk is half enclosed a small embryonic shield (area pellucida) makes its appearance in the centre of the blastoderm, which has, in the meantime, become divided into two layers. The upper of these is the epiblast, and the lower the hypoblast. The embryonic shield is mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. It is somewhat pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the narrow end an invagination takes place, which gives rise to an open sac, the blind end of which is directed forwards. The opening of this sac is regarded by the authors as the blastopore. A linear thickening of epiblast arises in front of the blastopore, along the median line of which the medullary groove soon appears. In the caudal region the medullary folds spread out and enclose between them the blastopore, behind which they soon meet again. On the conversion of the medullary groove into a closed canal the blastopore becomes obliterated. The mesoblast grows out from the lip of the blastopore as four masses. Two of these are lateral: a third is anterior and median, and, although at first independent of the epiblast, soon attaches itself to it, and forms with it a kind of axis-cord. A fourth mass applied itself to the walls of the sac formed by invagination.