218 PEOR W. Iv. PAEKER ON THE 



Fifteen oftliese vertebrae will be enclosed by tbe ilia, only ten are at present; the 9th 

 and 10th are the first two of the uro-sacral series. In front, the dorso-sacral region is that 

 which retains distinct ribs ; the Inmbo-sacral, those which have only "buttresses" — four 

 altogether. The true sacrals are four in number, and the hu^e sacral ganglia abort their 

 diapopliyses, which are high up and flat. Behind the true sacrals, at this stage, the cartilage 

 does not form a floor to the notochord, which is naked below. Thus the symmetrical 

 cartilaginous nuclei that grow over the myelon above and enclose the notochord below 

 are seen to be the homologues of the cartilaginous patches already referred to, that 

 appear, right and left of the myelon, in the Lamprey, during its metamorphosis. 



Thus at this stage there are fifteen cervical, five dorsal, fifteen general sacral, and, at 

 present, ten caudal ; the notochord projects considerably behind the last feeble pair of 

 nuclei, which are not perfectly chondrified. There are forty-five in all ; seventeen of 

 these are behind the sacral nei'ves, and therefore correspond to those of the tail in a 

 Reptile. 



IV. — Stage 2. Chick after 8| days' incubation. 



In this stage I have only figured the hinder part of the vertebral chain — the sacral 

 and caudal regions. The chondrification is rapidly becoming complete, but the arches 

 are imperfect above, and the notochord still projects behind (Plate XXIII. fig. 5). In the 

 dorso-lumbar end of this tract, the ribs on the 2nd, 3rd, and 4th, as well as that developed 

 on the 1st, are still distinct ; and those on the 1st and 2nd uro-sacrals are, as bars, sepa- 

 rated from the diapophyses, although they are not segmented off; a distinct osseous 

 centre afterwards appears in these lower bars. The notochord {n.c.) is still ichthyic, its 

 partial segmentation tending to become hourglass-shaped, the constrictions being inside 

 the centra. This stage gives us the best analysis of the sacral and caudal regions of the 

 adult. The dorso-lumbar region of the general sacrum has one developed and three 

 arrested ribs; then follow four more vertebrae that are the true sacrals ; these have no ribs, 

 and their diapophyses are seen to be high up ; they are dilated to support the broad hip- 

 plates, and scooped below, to make room for the large ganglia of the sacral nerves. Then 

 as the nerves are suddenly diminished in size, the ribs break out again on the first two 

 uro-sacrals, and the diapophyses growing outwards and a little backwards, gradually 

 pass, as to form and size, into the caudal series. There is, however, a break between the 

 uro-sacrals and the caudals, the segmentation being sharply defined behind the 7th uro- 

 sacral or 15th general sacral vertebra. The next five are alike, but gradually lessen in 

 size; they show no distinct intercentra such as exist in most birds. These are followed 

 by six pairs of rudiments that do not cover the whole of the end of the notochord [n.c.); 

 this series of fading vertebra? becomes the "ploughshare " or uropygial bone. 



V. — Stage 3. JEmhryo o/Phasianus versicolor, two-fifths rljpe; and of Common Fowl, 



10 days' incubation. 

 Among a large number of Gallinaceous embryos of various genera and species, one ot 

 Phasianus versicolor yielded me the clearest view of the cervical and dorsal vertebrae in 



