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M a y, 191 7 SOME FACTORS IN THE NESTING HABIT OF BIRDS 89 We may add to the above factors some others, and discuss the series as fol- lows: 1. Perhaps the first factor leading toward the care of eggs by the oviparous vertebrates is the reduction of the number of eggs in a clutch and the increase in the amount of yolk and food materials. This reduction in number, from. the hun- dreds or thousands of eggs laid by the lower vertebrates to the few laid by the average reptile or bird, makes posslble better care for the few. This decrease in number and increase in care already appears in the fishes. Here, however, the problem is complicated by the factors of en- vironment, for the highly specialized pelagic mackorels strew their thousands of eggs un-Var? cared for on the surface of the sea, while the more primitive catfishes, using the opportuni- ties afforded 'them by the stream beds, guard jealously in a nest the few hundred eggs they produce each season. The vertebrate egg, denuded of food and protective envelopes, is a single. cell, whleh in the chicken is but one twentieth of a millimeter in diameter. As such these cells occur in the ovary of the fowl. In the later development food in the form of yolk is added inside this cell until it may become more than an inch in diameter, then, during its p?ssage down the ovi- duct, there are wrapped about it the nutritive envelopes of albumen and the fibrous and cal- careous envelopes, which we know as the egg- shell. Fig. 36a shows the later development of the egg in the ovary, where yolk is being added to it, and the arrangement of the oviduct in which those parts other than the yolk are add- ed. Fig. 36b shows a hen's egg in section in which the eye of the yolk is the only living part. 2. Immediately associated with this in- crease in the amount of yolk material and the addition of the nutritive albumen is the devel- opment, first, of the fibrous, and then of the calcareous, shell. This hard shell was probably necessitated by the change of the reptilian an- cestors from a semi-aquatic to a purely terres- trim environment, for the eggs then required such an impervious shell to protect them from. dessication. As the results of a change are sel- Fig. 36. a. OVARY AND OVIDUCT IN THE CHICKEN, SHOWING THE EGGS OF TIIE'OvARY EACH IN ITS FOLLICU- LAR SAC WIIERE IT IS ACQUIRING YOLK. BELOW THE OVARY THE GLANDULAR OVIDUCT, WHICH, WHILE THE EGG PASSES TIIROUGH IT, SE' CRETES AROUND THE EGG THE 1VI{ITE AND THE SHELL. b. DIAGRAM OF A HEN'S EGG IN WHICH THE t'EYE" OF THE YOLK IS THE INCIPIENT EM- BRYO, THE ONLY LIVING PART OF THE EGG. dom simple, we find that the hard shell made possible more elaborate nests, and these placed in a greater variety of situations, as the developing embryo was then protected from mechanical injuries as well as from drying. 3. One of the greatest steps in advance towards arian nidifieation was the increase and stabilization of the body temperature. This probably occurred slowly as the birds became more and more differentiated from the reptiles. In- dications of this slow increase still remain, as in Apteryx and others of the lowest