178 



Mr. W. H. Caldwell. 



[Mar. 17, 



cells, travel thence to the ovum; food granules also appear in 

 the neighbourhood of the germinal vesicle, and travel away from 

 it : hence the horse-shoe shape of the yolk-mass as seen in section. 

 The time during which food granules are thus passing from the 

 follicular cells to the ovum maybe called " the yolk forming period." 



It is succeeded by a period during which the vitelline membrane 

 again becomes thin, the follicular cells are reduced to a single layer, 

 and the cells are very thin and flat. This period may be called 

 the " absorption of fluid period," since during it the ovum absorbs 

 large quantities of fluid through the thin vitelline membrane and 

 single layer of thin follicular cells, and thereby increases largely in 

 size. 



This is in turn succeeded by a third period, during which the folli- 

 cular cells again become active, multiply, increase greatly in size, and 

 give rise between themselves and the vitelline membrane to a deeply 

 staining homogeneous layer, which I will call the chorion. This period 

 may hence be called " the chorion forming period." All these three 

 periods are gone through while the ovum is still in the follicle. 



Upon the bursting of the follicle and the reception of the ovum in 

 the Eallopian tube, a few of the follicular cells remain attached to the 

 chorion ; the majority are left behind within the burst follicle. 



During the passage along the Fallopian tube, the vitelline mem- 

 brane again increases in thickness, and the chorion, also increasing in 

 thickness, absorbs fluid and becomes the albumen layer. Outside this 

 now appears a new structure, the shell or shell-membrane, of tough 

 parchment-like consistency,* not staining with reagents. I have not 

 yet traced the deposition of the shell to the activity of any special 

 glands ; but I can say that the shell-membrane does not increase at 

 the expense of the chorion or albumen layer. 



After reaching the uterus both vitelline membrane and shell-mem- 

 brane increase in thickness, but the albumen layer diminishes and 

 disappears, serving apparently for the nutrition of the ovum. Imme- 

 diately beneath the vitelline membrane a new layer is now seen in 

 hardened preparations ; but it may be shown that this layer is really 

 fluid, yielding a coagulum which stains deeply with reagents, the 

 fluid being apparently derived, through the membranes, from the 

 uterine glands. 



In Marsupialia the history of the vitelline membrane, save that 

 " the yolk forming period" is not marked off from the " absorption of 

 fluid" period, is similar to that in Monotremata. I have not been able to 

 trace the beginning of the " chorion " while the ovum is still in the 



* In the shell of the laid egg of Echidna I have not detected calcic salts, hut that 

 of Ornithorhyncus gives rise to gas when treated with dilute acid. 



