340 MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 



notwithstanding the different condition of the ovum of the bird when it reaches the 

 oviduct, it must be observed that the material to be employed in constructing the 

 embryo is derived, as in Mammalia, from the oviduct. It is the white of the egg 

 which disappears during incubation, while the greater part of the yolk is inclosed in 

 the abdomen of the chick at the conclusion of that process ; so that, although the 

 yolk has a prior existence to the albumen, and is generated in the ovary itself, it is 

 analogous in its function to the milk which nourishes the new-born mammal. 



Without, however, entering into the further uses of the yolk in birds, which affords 

 an admirable example of prospective design, it is sufficient for the present purpose to 

 observe, that while it affords the chief differential feature between the oviparous and 

 mammiferous ovum as this is first received into the oviduct, so a corresponding dif- 

 ference is manifested in the structure of the recipient tube as well as in the ovary 

 itself. 



Now the true Fallopian tubes of the Kangaroo closely resemble, both in relative 

 size and in structure, those of the ordinary Mammalia. The difference is manifested 

 in the greater proportional extent of the fimbriated extremity and its relations to the 

 ovary, which are circumstances in which the ordinary Mammalia also differ among 

 each other. 



From this accordance, therefore, and from the circumstance of the young being 

 nourished after birth by the secretion of mammary glands, it may be safely concluded 

 that the ovulum in the Kangaroo quits the ovisac in a condition corresponding to that 

 in the ordinary Mammalia, and increases in a similar manner as it descends to the 

 uterus. 



Additional evidence in favour of a correspondence in the original development of 

 the primordial germ in marsupial and ordinary Mammalia is derivable from the 

 structure of the ovary itself, and especially from its appearance after impregnation. 



The cellular structure or parenchyma of the ovary, in which the ova are developed, 

 is as dense in the Kangaroo as in the ordinary Mammalia. The cavity of the Graafian 

 follicle or ovisac from which the germ of the foetus above described had escaped, was 

 partly occupied by coagulated caseous substance, and partly by the irregularly thick- 

 ened membrane of the ovisac ; thus forming a corpus luteum=* as in other Mammalia : 

 this was of a large size in proportion to the rest of the ovary, and the external ori- 

 fice, probably from the abundance of the secretion as well as from the dense structure 

 of the ovarian capsule, had not become cicatrized. In Birds, on the contrary, owing 

 to the delicate and yielding nature of the stroma ovarii and from the tenuity of the 

 capsule of the ovary, permanent corpora lutea are not formed except under accidental 

 circumstances. 



* The corpus luteum in another Kangaroo, six months after impregnation, I found to be composed of a sphe- 

 rical body from two to three lines in diameter, of a pink colour and fleshy substance ; the membrane covering 

 this body was vascular, and the cicatrix had nearly disappeared. A corpus luteum, eighteen months after im- 

 pregnation, was of smaller size, did not project from the ovary, and was of a dark colour and firm texture. 



