liY D. emblp:ton, m.d. 57 



ova. Below this opening the oviduct is a tortuous but not long 

 canal, which ends at the cloaca and the vent. The oviduct is 

 formed of three layers ; an outer serous, or peritoneal ; a middle 

 or muscular, in two strata, the external of which has the fibres 

 running longitudinally, the intei-nal circularly ; and an internal 

 lining or mucous membrane, of great delicacy, and which varies 

 considerably both in structure and function in the three great 

 di"\dsions of its extent. 



An ovum discharged into the open end of this canal is simply 

 the yolk, enclosed in a pellucid membrane of extreme fineness, 

 its other coverings being gradually laid on in succession as it 

 passes on to the lower part of the oviduct. 



The best account I have seen of this passage of the ovum, 

 in the case of the Common Fowl, is that by Professor Allen 

 Thomson, in his highly interesting article " Ovum,^^ already re- 

 ferred to, from which I quote the following paragraphs. 



" The passage of the yolk through the first two-thirds of the 

 length of the oviduct, in which part the albumen is deposited, is 

 very rapid, scarcely occupying more than three hours, according 

 to Coste, before it arrives at the narrow or constricted part of a 

 more limited extent (isthmus) in which the membrane of the 

 shell is formed. 



''About three hours more suffice for this process, and the 

 ovum then enters the dilated portion, which has been called 

 uterus, in which the substance of the shell is deposited and gra- 

 dually consolidated on its surface. 



' ' The egg does not descend in a straight, but in a spiral, di- 

 rection, corresponding with that of the ridges of glands with 

 which the mucous membrane of the upper part of the o^'iduct is 

 beset, and the various layers of albumen are deposited spirally 

 one over the other. It has been ascertained by experimental 

 observation that the membrane of the shell is formed in the 

 narrow part of the oviduct, which intervenes between the 

 albuminiferous part and the uterine dilatation. It consists, no 

 doubt, in the fibrillation of consolidated albumen or some ana- 

 logous substance, which must take place with great rapidity, but 

 we are not yot sufficiently acqnaint(Ml witli the nature of this 



