THE OVUM OF THE BIRD. 943 



come insufficient for the nourishment of the embryo, and special struc- 

 tures are very early formed to enable it to derive support from the 

 nutrient fluids of the parent. These are imperfectly developed in the 

 Implacental, but are more complete in the Placental Mammalia. .Of 

 the meroblastic ova, those of Reptiles and Birds, but particularly the 

 latter, present by far the most abundant nutritive yolk. From the siz,e 

 of the egg and from the occurrence of all the stages of development 

 during an external incubating process, which may be carried on by 

 artificial means, the egg of the Bird especially presents the most f,vor- 

 able opportunity of watching, from hour to hour, the stages of develop- 

 ment of the Vertebrate embryo, within the ovum. 



The Ovaries and Ova of the Bird. 



The egg of the common fowl is first formed within the body of the 

 hen, in the organ named the ovary, which is attached to the back of 

 the abdominal cavity, in the lumbar region. In the female embryo of 

 all Birds, two ovaries exist, but almost universally the left one only is 

 present in the mature Bird; the Dorking fowl is an exception, having 

 both ovaries persistent. This is also the case in certain of the birds 

 of prey. The ovary itself consists of a cluster of small spherical bodies, 

 closely invested by membranous ovisacs, and named the ova. These 

 are at first destitute of the white, and consist only of minute yolks 

 invested with the vitelline membrane. The ovisacs are all held together 

 by a loose areolar stroma and bloodvessels, so as to form a bunch or 

 raceme, and are invested by the peritoneum. To the lower part of the 

 ovary, is attached the wide funnel-shaped opening, or infundibulum, 

 of a long tortuous tube, named the oviduct, which is also single, being 

 present only on the left side; it opens below into the cloaca, or common 

 outlet of the alimentary, urinary, and reproductive organs in the Bird. 

 In the ovary, each yolk is inclosed in its ovisac, the narrow suspensory 

 part of which is named the pedicle. The yolks are of all sizes, from 

 that of a pin's head, or smaller, to the completely-formed yolk. In 

 structure, the minute ova at first resemble the holoblastic ovum ; but 

 as they grow, they become meroblastic, and the cicatricula, or disc 

 (Fig. 117) of the germ-yolk, or formative yolk, is very early recognized 

 upon the rapidly increasing formative yolk ; it is nearly always at that 

 part of the yolk which corresponds with the pedicle of the ovisac. It 

 is now that i\\e germinal vesicle, v,g, with its contained sp'ot, or macula 

 germinativa of Purkinje, are distinctly seen; but no nucleated cells 

 exist. The vesicle and spot disappear as the yolk descends along the 

 oviduct, whether the egg be fertilized or not ; they are not found when 

 the egg is laid, Fig. 118, but the cicatricula has then become subdivided 

 into two layers, the deeper one containing nucleated cells, many of 

 which are also seen in the central parts of the yolk. As each yolk 

 enlarges, its ovisac increases in vascularity, and, when the yolk ap- 

 proaches maturity, a non- vascular band, or zone, forms around it, in 

 which, at a part named the stigma, a rupture occurs, and the yolk 

 escapes into the infundibulum of the oviduct. The remainder of the 

 ruptured ovisac, with its coverings, is cup- shaped, and forms the calyx, 



