OP MONOTREMATA AND MARSUPIALIA. 
477 
This vitelline membrane was first noticed by Valentin ; Barry (60) figured it 
in 1838, but Bischofe (66) immediately afterwards explained that Barry and the 
previous observers had been misled by a line caused by the existence of cilia round the 
young ovum. 
H. Meyer (72), Reichert (73), and Van Beneden (63) have distinguished this 
structure, and regarded it as a true vitelline membrane. He ape (70) found and 
figured it in sections of the ripe ovum of the Mole. 
Barry stated in his papers that it disappeared by liquefaction, but in a foot-note in 
p. 338 in his last paper he says that it “ may perhaps contribute to the thickening ” 
of the zona pellucida; and this foot-note of Barry’s is evidence confirmatory of the 
suggestion that I now make concerning this membrane. 
Van Beneden (64) says the appearance of this true vitelline membrane coincides 
with maturation and the formation of polar bodies. 
I would suggest, therefore, that it is the homologue in Placentalia of the layer which, 
appearing during maturation as a line containing granules, becomes the coagulum of 
later stages in Monotremata and Marsupialia. 
Von Baer (58) discovered the zona pellucida in the ovary. Barry (62) figured its 
radial striation, caused by the pointed ends of the follicular epithelium, and its 
granular outer layer. 
In 1854 Remak described the radial striation, but it was not until 1870 that 
Waldeyer (74) called attention to the distinction between the outer and the inner 
layers of the zona radiata. 
Waldeyer considered the granular layer a product of the follicular epithelium. 
Balfour (59) put forward a curious hypothesis with regard to this outer layer. 
He considered that it was the remains of the vitelline membrane surrounding young- 
ova. Balfour’s hypothesis was rendered necessary by his comparison of the inner 
zona radiata of Mammalia with the transitory appearance which he called zona 
radiata in Elasmobranchs. 
Van Beneden (65), p. 514, pointed out Balfour’s mistake, and said, “It is certain 
that the membrane which Balfour called vitelline membrane in the Rabbit is the 
zona radiata, and not its outer granular layer.” There is, therefore, in Placentalia, as 
in Monotremata and Marsupialia, a delicate membrane round the young ovum, which 
becomes the thick membrane known as the inner homogeneous layer of the zona pellu¬ 
cida. This membrane is the true vitelline membrane of Placentalia. The outer 
granular layer may therefore be interpreted as the homologue of the pro-albumen. 
The outer covering of the uterine embryo was discovered by de Graaf in 
1692. Von Baer (58) suggested that this “ villous chorion” ( Schalenhaut ) was directly 
derived from the zona pellucida of the ovary. 
Coste and Rudolf Wagner held the same view; but Purkinje, Valentin, and 
Allen Thomson all considered that the chorion probably arose in the oviduct, like 
the egg-shell of other Vertebrates. 
