BIRDS. 183 



clavicle and inter-clavicle grow (see figs. 6 and 7, cl., i. cl., d. p. cr.), is the counterpart of the 

 right and left " distal prae-coracoids," which are so frequently developed in the Mammalia. 1 



At this stage the Sternum (fig. 2) is very instructive ; it is almost divided by deep notches 

 into six sub-parallel bands of cartilage. The broadest as well as the longest bands meet at the 

 mid-line. They (e. s.) are separated by the primordial ventral fissure, which is rapidly disap- 

 pearing ; it has, however, left an emargination in front and a long slit behind. Each half of this 

 double band, nearer the end than the front, has sent down a delicate lamina ; these laminae have 

 coalesced below ; the result is the sternal keel. A section taken towards the front of the rudi- 

 mentary keel (fig. 3, k.) shows the primordial fissure, not yet filled in, above. The lower coracoid lip 

 (fig. 2, cr. g.) is seen to be an " outgrowth" arising behind the inner margin of a large deep notch, 

 separating the rostrum (r.) from the costal process (c. p.) ; a moderately broad isthmus separates 

 this front notch from the large open cleft which divides the " intermediate" from the " middle 

 xiphoid" (i. x., m. x.). Another notch, two thirds the size of the former, separates the outer (e. x.) 

 from the intermediate xiphoid. The external xiphoid diverges very much from the other, and is 

 largely pedate, looking fowards and upwards, and embracing the ribs. In front of the outer 

 xiphoid, and mounting up the costal process (c. p.), there are four costal condyles (c. c.). The whole 

 of this multifid Sternum of the embryo Pheasant is, and will be for some days to come, entirely 

 soft. A day later, in the chick of the common Fowl, a much larger mass of meso-scapular hyaline 

 cartilage (fig. 9, m. s.c. s.) can be seen receiving bony matter from the clavicle (cl.) ; and in 

 fig. 8 it is seen that the clavicles have begun to coalesce, and that the inter-clavicle (i. cl.) is much 

 smaller than in the Pheasant ; it is imbedded in a delicate mass of nbro-cartilage the distal prse- 

 coracoid (d. p. c. r.). The next important stage in the Sternum is shown in the common Chick, 

 two or three days after hatching (figs. 10 and 11 magnified three diameters; fig. 11 is a side 

 view of the Sternum of a chick a day or so older than that shown in fig. 10) ; here we have the 

 five bony centres that appear in the typical Gallinaceous Birds. 



These osseous patches appear earlier, are much more definite in outline, and disappear later 

 than in Birds generally, the reason of this being that they are ectostoses; and this mode of 

 ossification of the Sternum has not been seen since we first came across it amongst the higher 

 tailless Batrachians, for instance, Rana and its nearest congeners. The five centres are synchronous 

 or nearly so; each costal process (c. p.) and region (c. c.) is ossified by a pleurosteon (pi. o.), 

 the strongly-forked outer xiphoid (e. x., i. x.) by a metosteon (m. o.), and all the ento-sternal 

 region, from the rostrum to the middle xiphoid by the lophosteon (1. o.). A section through the 

 anterior part of the lophosteon shows (see fig. 12, magnified ten diameters,) the dense ectosteal 

 layer, which is growing inwards, and is seen to fill up the (primordial) fissure above, and to be 

 creeping down the cartilage (c.) of the keel below. The general form of the Sternum has 

 changed in some respects ; the two coracoid lips (cr. g.) are more nearly equal ; the more 

 extended rostrum has acquired a fenestra (r. f.), which partly cleaves it from the rest of the ento- 

 sternum. I have already shown that this cleft appears in Upupa epops and in Steceros albirostris. 

 The intermediate xiphoid (i. x.) has acquired a " foot," and the middle bar (m. x.) is now one 

 piece ; the keel (k.) is deeper, and it sends forwards a sharp hook at its angle ; the keel 



This is only one among many instances in which the Bird adumbrates the Mammal, rising 

 indeed to almost the same morphological height ; and only lying obliquely between the Reptile and the 

 Mammal. 



