Some Observations on Vitally Stained Rabbit Ova with Special Reference to their Albuminous Coat

T H E gradual deposition of a homogeneous, jelly-like covering on the rabbit ovum during its passage down the uterine tube was first observed by Cruikshank in 1797. This observation has been confirmed repeatedly by many later workers (Barry, 1839; Bischoff, 1842; Thomson, 1859; Assheton, 1894; Gregory, 1930; and Pincus, 1936). Barry referred to this covering investing the zona pellucida as 'chorion', some called it the 'pro-chorion', while others (Lenhossek, 1911; Grosser, 1927) simply spoke of a deposit of mucus due to the glandular activity of the tubal epithelium. In the more recent literature (Gregory, 1930; Pincus, 1936) this layer is generally referred to as an 'albuminous' coat, probably because of its slight similarity to the covering of a hen's egg. Lenhossek (1911) described a scanty but an identical coat on the ovum of the horse and dog. Hamilton & Day (1945) found a thin coat of material on the tubal ovum of the horse similar to that on the rabbit ovum. In monotremes (Hill, 1933; Flynn & Hill, 1939) and in many marsupials (Hartman, 1916; Hill, 1910, 1918; McCrady, 1938) a thick albuminous coat is laid down on the zona pellucida as the egg traverses the oviduct. The investigations of Hammond (1934) on the fertilization of rabbit ova in relation to time of insemination show that the albuminous coat deposited on the tubal ovum plays an important part in preventing fertilization. While the early terms 'chorion' and 'pro-chorion' expressed a certain mistaken belief in the probable functional significance of this albuminous covering of the rabbit ovum, until recently neither the true chemical nature nor the probable role played in the physiology of the developing egg by this coat has been the