I.] DESCENT OF THE OVUM. 21 



into the cloaca. The whole of the mucous membrane lining 

 the oviduct is largely ciliated. 



The accessory parts of the egg are entirely formed in the 

 2nd and 3rd portions. The layer of albumen which imme- 

 diately surrounds the yolk is first deposited; the chalazae are 

 next formed. Their spiral character arid the less distinctly 

 marked spiral arrangement of the whole albumen is brought 

 about by the motion of the egg along the spiral ridges into 

 which the interior of the second or tubular portion of the 

 oviduct is thrown. The spirals of the two chalazae are in 

 different directions. This is probably produced by their 

 peripheral ends remaining fixed while the yolk to which 

 their central ends are attached is caused to rotate by the 

 contractions of the oviduct. During the formation of the 

 chalazae the rest of the albumen is also deposited; and 

 finally the shell-membrane is formed in the narrow neck 

 of the 2nd portion, by the fibrillation of the most external 

 layer of albumen. The egg passes through the 2nd portion 

 in little more than 3 hours. In the 3rd portion the shell is 

 formed. The mucous membrane of this part is raised into 

 numerous flattened folds, like la,rge villi, containing follicu- 

 lar glands. From these a thick white fluid is poured out, 

 which ' soon forms a kind of covering to the egg, in which 

 the inorganic particles are deposited. In this portion of the 

 oviduct the egg remains from 12 to 18 hours, during which 

 time the shell acquires its normal consistency. At the time 

 of laying it is expelled from the uterus by violent muscular 

 contractions, and passes with its narrow end downwards 

 along the remainder of the oviduct, to reach the exterior. 



13. We have now to trace out the changes which take 

 place in the germinal disc, during the passage of the egg 

 down the oviduct. 



By the time when the egg becomes clasped by the expanded extremity of 

 the oviduct the germinal vesicle has, according to Oellacher (loc cit. and also 

 Archiv. fur Micr. Anat. Vol. vin. 1872. p. 18), undergone still further 

 retrogressive changes. It has now become very much flattened and closely 

 applied to the vitelline membrane. Both this and former stages, if we may 

 judge from the analogy of osseous fishes, are preparatory to the whole germinal 

 vesicle being bodily ejected from the germinal disc. For further particulars 

 vide Oellacher, Archiv. fur Micr. Anat. Vol. vill. pp. i 26. 



Impregnation occurs in the upper portion of the oviduct ; 

 the spermatozoa being found actively moving in 'a fluid which 

 is there contained. 



