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 diiring 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 Laridae and Limicolse (the affinities of which 

 two groups it took Ornithologists a long time to find out), between 

 Picidce and Passeres, Striges and Caprimulgidge, 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 ovum 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 



