BULLETIN 255.

DWARF EGGS OF THE DOMESTIC FOWL.'

By RAYMOND PEARL AND MAYNIE R. CURTIS.

In any breed of domestic fowls there are occasional individuals which produce one or more small eggs which are from one-tenth to one-half the size of normal eggs. Various superstitions have been associated with these small eggs and in many parts of the world the eggs are still called by names derived from the superstitions formerly common to the region. A common superstition which persisted nearly to our own time was that old cocks produced these eggs. Hence they are still often called "cock eggs." At an early period it was believed that if a cock egg was incubated it would hatch into a serpent, the basalisk, whose breath or look was fatal. Other less definite superstitions considered these eggs as good or bad omens. In some places they were used as charms to bring misfortune to one's enemies and were called "witch eggs." In other places they were called "luck eggs" for it was believed that if one was thrown over a building any wish made by the thrower while the egg was in the air was sure to come true. Since no one of these names is generally accepted we have decided to use the modern descriptive term "dwarf eggs."

The dwarf egg is more common than any other type of abnormal egg except the double-yolked egg. It has emerged from the age of superstition with the cause for its production inadequately explained. It is the purpose of the present paper to discuss (1) the different types of dwarf eggs in respect to shape and also in respect to contents; (2) the variability in respect to size and shape; (3) the interrelations of the varia

"This bulletin is an abstract of a more detailed paper by the same authors published under the title "Studies on the Physiology of Reproduction in the Domestic Fowl. XV. Dwarf Eggs." Journal of Agricultural Research, Vol. VI, pp. 977-1042, 1916.

tions in dimensions, shape and size; (4) the frequency of the occurrence of dwarf eggs compared to normal eggs and of dwarf egg producers compared to birds which do not lay dwarf eggs; (5) the seasonal distribution of dwarf eggs; (6) dwarf egg production by birds with normal and with abnormal oviducts; (7) the relation of dwarf egg production by normal birds to the age of the bird and to the position of the egg in the clutch and litter; (8) physiological conditions which lea i to dwarf egg production; (9) the relation of the production of dwarf eggs to other abnormal phenomena of reproduction which either occur in nature or have been experimentally produced; and (10) the contribution which the study of the physiology of dwarf egg production makes to our knowledge of the normal physiology of egg production.

In the eight years from February 1, 1908, to February 1, 1916, 298 dwarf eggs are known to have been produced at this plant. The weight of 275 of these was taken and in 261 of these cases the length and breadth was also measured and the length-breadth index calculated. Of the 298 eggs recorded 274 were opened and their contents were examined. Several of the dwarf eggs were floor eggs and a few were laid by birds on which no egg record was kept. In 251 cases, however, the egg record of the bird laying the dwarf egg is available. Further several of these birds were autopsied and the condition of their sex organs observed.

I. DIFFERENT TYPES OF DWARF EGGS CLASSIFIED, FIRST, IN RESPECT TO SHAPE AND, SECOND, IN RESPECT TO PRESENCE OR ABSENCE OF YOLK.

The dwarf eggs of the fowl vary greatly in size and shape. Fig. 51 shows fourteen of these eggs with a normal egg laid

by a nine months old pullet for comparison. From this photograph it may be seen that there are two distinct types of dwarf eggs in respect to their shape. The prolate-spheroidal type similar in shape to a normal egg and the cylindrical type which is much longer in proportion to the breadth. The cylindrical eggs are shown in the first column of Fig. 1. These cylindrical eggs occur much less frequently than do the dwarf eggs of the prolate spheroidal type.

Not only do the dwarf eggs differ in respect to size and shape, but there is a difference in internal structure. Some of these eggs contain no yolk but appear to be formed around a nucleus which consists of a few strings of coagulated albumen, apparently untwisted chalazal threads, and there are also sometimes small lumps of hardened albumen or small blood clots associated with these chalazal threads. Some dwarf eggs contain small yolks in yolk membranes. More than half of all the eggs opened, however, contained some yolk which was not enclosed in a yolk membrane. Dwarf eggs may then be classified according to the non-occurrence of yolk and the condition of the yolk when present as, first, yolkless, second, with some yolk not in a membrane and, third with one small yolk. In Table 1 the dwarf eggs are classified both according to form and yolk content.

TABLE I.

Showing the Classification of Dwarf Eggs both as to Shape and as to Yolk Content.

From the last line of Table 1 it is seen that 96, or 35.03

per cent. of the dwarf eggs opened were yolkless. The other

178, or 64.96 per cent. contained yolk. Of these, 151, or 55.11 per cent. of all the dwarf eggs opened contained yolk not enclosed in a yolk membrane. A small yolk was present in 27, or 9.85 per cent. of the dwarf eggs. From these figures it is seen that nearly two-thirds of the dwarf eggs contain yolk.

II. THE ALBUMEN AND SHELL of Dwarf Eggs.

We have seen that dwarf eggs differ in respect to the nucleus around which the albumen is formed. Bonnet states that the nature of the albumen is also generally altered. The dwarf eggs observed differed greatly in respect to the density of the albumen. In many it was very condensed, being a thick clear mass which nearly maintained its shape when removed from the shell and egg membranes. It appeared very much like the albumen in a normal egg while it is in the albumen secreting region, or the isthmus of the oviduct. In many other cases it appeared exactly like the albumen of a norma! laid egg, that is, there was a somewhat firm inner mass surrounded by a thin fluid albumen. All gradations between these also occurred. In a very few cases the albumen was more fluid than in the average normal egg. There, however, was an undoubted general tendency for the albumen to be more than normally firm. In connection with another investigation in progress at this laboratory the specific gravity of the albu men of many normal and a few dwarf eggs was determined. The specific gravities of the dwarf eggs ranged widely, with the upper end of the range decidedly above the range for normal eggs. In fact the mean for the dwarf eggs is higher than the maximum for normal eggs, while the minimum for dwarf eggs is only slightly below the mean for normal eggs.

The egg membranes of dwarf eggs so far as superficial appearance indicates are comparable to those of normal eggs. The shell is sometimes entirely or almost entirely absent as in the case of membrane covered or soft shelled eggs which

'Bonnet. 1883. Das Vogelei. Deutsche Zeitschrift für Thiermedicin. Vol. 9, pp. 239-252.

'Pearl, R. and Curtis, M. R. 1912. Studies on the Physiology of Reproduction in the Domestic Fowl. V. Data Regarding the Physiology of the Oviduct. Jour. Expt. Zool., Vol. 12, pp. 99-132.