DEVELOPMENT OF MONOTREMATA, 



715 



Ovum from uterine end of the oviduct, with, the 

 addition of a layer of albumen, Rabbit ; magn. 

 350 diam. cclxi". 





mass of finely nucleate corpuscles; amongst which the qualities 

 of the parent embryo-cell, due to impregnation, are thus equally 

 distributed. 



The eight- fold cleavage of the yolk has been observed three days 

 after impregnation in the Rab- 

 bit, four days in the Guinea- 

 pig, and ten days in the Bitch : 

 always in ova toward the ute- 

 rine end of the Fallopian tube. 



In the Bitch the smooth 

 surface of the zona pellucida 

 becomes irregularly flocculent, 

 as if a granulo-mucous sub- 

 stance had been deposited 

 thereon: in the Rabbit the 

 ovum acquires a thick adven- 

 titious layer of albumen, fig. 

 565, a, before entering the ute- 

 rus : in the Guinea-pig the 

 zona continues smooth ; and, 

 after entering the uterus, on the fourth day, it grows fainter as 

 the mulberry state of the yolk is there attained, and it disappears 

 when the germ-mass is completed. The act of impregnation being 

 thus consummated, ulterior changes with manifold modifications 

 attend the development of the ovum in different Mammalia. 



§ 399. Development of Monotremata. — The ripe ovarian ovum, 

 though large in proportion to that in higher, especially placental, 

 Mammals, is very much less than in Birds or Reptiles. Its 

 external coat is thick, smooth, highly refracting — a true 4 zona 

 pellucida ' : the germinal vesicle is ^^th of an inch in diameter : 

 the larger proportion of vitelline matter, rich in granules and 

 oil globules, is the chief distinctive character of the mono- 

 trematous ovum as a Mammalian one. I found two ovisacs 

 with such mature ova in the left ovary of a female Ornithorhyn- 

 chus, killed in September. In a specimen killed on the 6th 

 of October (Yas River, New South Wales), the left ovary pre- 

 sented two discharged and altered ovisacs. The ova from these 

 were situated at the upper part of the left uterus, and at the 

 distance of about a line from each other. Each was spherical, 

 and measured two lines and a half in diameter ; the germ -mass, 

 originally pale, had deepened to a yellow colour in the preserving 

 liquor. The outer tunic had received no adventitious covering, 

 but retained its smooth and polished exterior, and had not con- 

 tracted any adherence to the uterine parietes. Each ovum was 



