EMBRYOLOGY 73 



except by incubating it and awaiting developments in the embryo. Among eggs from wild 

 grouse the proportion hatching successfully has been consistently high and variations 

 apparently have had little relationship to fluctuations in the number of birds. Among those 

 from captive grouse the proportion has been somewhat less but the difference has been slight. 



Livability 



Will the chick live? This is a more difficult problem. The high mortality rate of the 

 young birds even after a successful hatch has been a primary factor in regulating the size of 

 fall populations. Yet what causes it cannot be determined from field data alone. Both the 

 quality of the egg and the environment into which the chick is born, play major roles. Labora- 

 tory and field research must be coordinated in order to find out what those roles are. 



Realizing this, the Investigation undertook a five-year study, from 1936 through 1940, to 

 determine, as far as possible, how much the inherent qualities of the egg influenced the 

 growth of the embryro and the viability of the chick. In all. 398 wild ruffed grouse eggs 

 from 44 clutches, collected near Ithaca, N. Y., and 189 newly hatched chicks were analyz- 

 ed. 



The study included the general development and structure of the eggs, their physico-chem- 

 ical properties, annual changes in quality, develojiment of the embryo and embryonic mem- 

 branes, and embryonic mortality. It also included the chemical composition of hatched 

 chicks and its annual changes. 



To date, the Investigation has not carried on extensive enough studies of the rest of the prob- 

 lems to report findings. The influence, if any. of climatic or solar changes on concentration 

 of essential food ingredients, as minerals and vitamins in plants and even insects, the effects 

 of food supply on egg development, how weather reacts on the quality of the eggs, and the pos- 

 sibility that variations in egg conformation may retard growth of the embryo and help deter- 

 mine livability, are still problems for the future. 



Development of Grouse Eggs 



Grouse eggs in their development resemble in many respects those of other gallinaceous birds, 

 such as pheasant and quail. The incubation period is normally about 23.5 days. It is only a 

 few hours shorter than that ol ])heasaiits and about one day longer than that of quail"". 

 From the weight of eggs and birds at hatching, it is also evident that the growth rate of the 

 grouse embryo is somewhere between the growth rates of pheasant and quail embryos'"". 



Structure of the Egg 



In structure the fresh grouse egg is, in general, similar to the eggs of other birds. It 

 consists of yolk, albumen, shell and blastoderm ( figure 41 . The stratification of yellow- and 

 white yolk corresponds to the day and night growth of the yolk in the ovary. The four 

 layers of albumen, with two cord-like attachments at the opposite poles of the yolk, oc- 

 cupy nearly two-thirds of the entire egg. except for a small space at the large end known 

 as the air-sac. Tlie inner surface of the egg is lined with two parchment-like membranes. 

 The blastoderm, appearing like a small whitish speck on the upper surface of the yolk, is the 

 seat of the future embryo. 



Physico-chemical Properties of the Egg 

 Table 8 gives the average total weight, shape, breaking strength, weight and thickness of 



