70 THE SPFXIES^ITS TAXONOMY, RANGE. BIOLOGY. & ECONOMIC IMPORTANCE 



Development of the Embryo 



The development of the grouse embryo is essentially identical with the development of that 

 in other birds""- ''". The structural complexity has a direct relation^hi]) to the environ- 

 mental conditions under which birds' eggs develop. Provision is made in the structure of 

 the egg for the protection of the embryo, for its nutrition, respiration and excretion. To serve 

 these important functional activities of the embryo there are present several secondary em- 

 bryonic structures termed "fetal membranes." Among the most important of these are the 

 yolk sac. allantois and anniion I figure 4). These organs apjjear earl\ in the life of the em- 

 bryo and persist until the time of hatching. 



The yolk sac, consisting of the yolk mass itself, is a chief source of food material for em- 

 bryonic growth. Shortly before hatching, an unused portion of it slips through the navel 

 into the body cavity and is absorbed during the first few days after hatching. The allantois 

 ultimately underlies the entire shell and serves as the functional "lung" of the embryo. The 

 allantoic cavity serves as a reservoir for the excreta of the embryonic kidneys. The amnion 

 surrounds the embryo and. bv its fluid, protects the embryo agaitist all mechanical shocks and 

 jars. 



Stud) of the development of yolk sac and allantois — because of the limited supph of eggs 

 available — was too incomplete to show any changes associated with grouse abundance. It 

 had been anticipated, in analogy with the other forms, that the allantois and yolk sac might, 

 in some cases, be undeveloped, causing the general symptoms of anemia of the embryo which 

 may result from a deficiency of vitamin E. Ordinarily the egg yolk is the carrier of this vita- 

 min. Similarlv it was expected that a deficienc) in vitamin D might be responsible for slow 

 calcification of the embrvo. Evidentlv this was not the case, as will be shown later from the 

 analyses for calcium of newl\ hatched grouse. 



Embryonic Mortality 



The average embrvonic niortalitv of all wild grouse eggs, incubated at the Laboratory, var- 

 ied from year to year. The highest was 32 per cent in 19.57. The reasons f<ir tliis arc not 

 known at present. 



Chemical Composition of \kwi.y H\T(:hi:i) Croi se 



The average values for the chemical composition of newh hatched grouse in terms of dry 

 weight, and total ash and calcium content, are shown in table 9. These values, in pro|)ortion 



TM'.i.i-; 0. (.Hi-:\ii<\i. COMPOSITION OK ii!<i m:\vi.v n\r<:iiKi) cnorsi-: ciiicks 

 coi.i.Kf 'n;i) i-no\i tiii-; rni vca. m:\v ^ou^. hi:(;ion i'i:!(>-i')io 



to the body weight, are comparable to those of other game birds, especially jiheasants and 

 quail. On the other hand, the pro])ortion of unabsorbed yolk to the body weight at hatch- 



