EMBRYOLOGY 195 
incubation the embryos of the most different Birds still so much 
resemble each other that the want of extensive examination need 
not be so much deplored. Towards the end of the first week 
internal and external differences appear, characteristic of the Order 
and Family to which the bird belongs, while, with exceptions, the 
generic differences make their appearance during the second week : 
specific difference can hardly be expected in the embryos. Of course 
the seven days’ embryo of a Sparrow is more advanced than 
that of a Duck, which requires four times as many days, or than that 
of an Ostrich, which requires more than seven weeks of incubation, 
but their several characteristic features can be discerned at the end 
of the first third of the whole period of incubation. A comparative 
treatise on avine embryology which is to render valuable taxonomic 
results will have to restrict itself to the latter half of the embryonic 
stages. Such a treatise is still a desideratum, and cannot be under- 
taken until a large, well-preserved, well-named, and well-timed 
material of embryos of a great number of any birds is at hand. 
Prof. Fiirbringer has incidentally drawn attention to the probably 
considerable help which may be derived from the resemblances 
between middle-aged embryos of certain Families, before their 
specialized forms of bill and feet are fixed, and then rather obscure 
the affinities of the Birds in question. He mentions the striking 
similarity between Laridz and Limicole (the affinities of which 
two groups it took Ornithologists a long time to find out), between 
Picide and Passeres, Striges and Caprimulgide, and so on. Very 
young nestlings of Humming-birds, kindly sent to me by Col. Feilden 
are scarcely distinguishable in general appearance from young Swifts, 
because their bills are still quite short and broad. 
Formation of the Ovum in the Ovary.—Each ovum is a globular 
yellow body, consisting mainly of yellow and white yolk, and sur- 
rounded by the follicular membrane, which is the bulged-out 
continuation of the stroma of the ovary. This membrane con- 
_tains numerous blood-vessels, through which the ovum is nour- 
ished and enabled to grow. Gradually the growing ovum draws 
the follicular capsule out into a stalk, surrounds itself with the 
vitelline membrane, and ultimately bursts the capsule, whereupon 
it falls into the body-cavity, or rather into the wide funnel-shaped 
mouth of the oviduct. ‘The stalk and rest of the burst capsule 
shrivel up, and are gradually absorbed without forming a corpus 
luteum, as is the case in Mammals. The ovuin is now ripe and ready | 
for fertilization. It shews the following composition: A small 
amount of white yolk, consisting of small vesicles with albuminous 
matter, and a number of globular highly-refractive bodies, forming 
a small mass at the centre of the ovum, and continued to the sur- 
face by a stalk expanding into a funnel-shaped disk, the edges of 
which are continued over the surface of the ovum as a delicate 
