96 Proceedings of the Royal Physical Society. 



object-lesson in variability. The most remarkable form which I have to 

 show consists of a small oval giving origin to a twisted tube like the horn of 

 a shorthorn sheep. Such occurrences have been in some measure cleared up 

 by a recent daring experiment made by Pearl and Surface {Science, xxix., 

 1909, pp. 428-9). In order to determine the nature of the stimulus which 

 induces the making of a shell, they performed an operation on a hen as a 

 result of which the contents of the intestine were made to pass through the 

 shell-secreting part of the oviduct. The interesting result was, that they got 

 curious enshelled masses of various shapes, and they were led to the 

 conclusion that the stimulus which excites the shell-making glands is 

 mechanical rather than chemical in nature, and that the formation of the 

 shell is brought about by a strictly local reflex, and is not immediately 

 dependent upon the activity of other portions of the reproductive system. 



It may be said that this was to a certain extent known before from a 

 study of what are popularly known as " wind-eggs." These are not true 

 ova, they contain no vitellus. They consist of a mass of albumen the 

 stimulus of which has induced the making of an enveloping shell. It has 

 also been shown that foreign bodies may ascend from the cloaca into the 

 oviduct and become surrounded by a shell. Thus a species of Distomum, 

 which frequently occurs in the bursa Fabricii opening into the cloaca, may 

 pass up the oviduct and be included intact in the albumen of an egg. 



In an interesting paper on abnormal eggs in fowls, J. Kunstler points out 

 {Me'm. Soc. Sci. Bordeaux, in., 1903, pp. 65-72, 7 tigs.) that a frequent cause 

 is a lack of tone in the oviduct, the normal peristaltic movements being 

 disturbed in consequence. Thus an egg may return on its path and become 

 surrounded by a second shell, thus resulting in one form of the not unfamiliar 

 ovum in ovo. 



In a paper on " Ovum in Ovo," by F. H. Herrick {American Naturalist, 

 xxxiii., 1899, pp. 409-414, 3 figs.), it is suggested that an abortive egg or 

 egg-fragment may be enshelled and then included within the shell of 

 another egg of larger size. In other cases, what is included has nothing 

 of the nature of an egg about it, though it has been enshelled. Entirely 

 different again are eggs with double or triple yolks, where we have to deal 

 with a fusion of the albumen in two or more ova, which are treated in the 

 oviduct as one egg and surrounded by a single shell. This process 

 may be sometimes complicated by the inclusion of a third egg of normal 

 size and already covered by a hard shell. So when we speak of an egg 

 within an egg, we may mean one of three or four different things. 



The problem of the factors which determine the shapes of the eggs of 

 birds has been discussed in a very interesting paper by Prof. D'Arcy W. 



