506 TRANSACTIONS OF SECTION D. 
3. The Germinal Disc in Naturally Incubated Eggs of Passer 
domesticus. By C. J. Patten, Sc.D. 
Due reflection on the facts that nests (or, in the case of those birds which 
make no nests, the soil on which the eggs are deposited) vary to an extra- 
ordinary extent in their heat-retaining properties ; that the protecting egg- 
shells vary strikingly not only in their thickness but in their porosity and 
other structural peculiarities; and lastly that avian embryos vary to a 
considerable extent as regards their vitality when heat is withdrawn from 
the shell, has led me to think that the method of studying avian embryology 
by means of the artificial incubator is not always the most reliable. In 
making this statement I am fully sensible of the successful issues which 
have been brought about by artificial incubation. Not only the eggs of the 
Domestic fowl, but also those of wild birds of markedly different nesting 
habits can be hatched out so that the chicks are healthy and continue their 
post-natal growth vigorously. Still, when using the incubator extensively 
one frequently finds that monstrosities arise as well as ill-defined develop- 
mental phases. That birds allow their eggs to cool down for short periods 
I have little doubt, this taking place in some species much more than in 
others. Furthermore I have noted how some birds are more anxious than 
others to return to the duty of hatching, due allowance being made in the 
case of close sitting towards the end of incubation. It would thus appear 
that brief periodic coolings of incubating eggs in a state of nature might 
prove beneficial rather than otherwise, and in support of this theory I 
would state that in my experience with the incubator I have found many 
more irregularities in the developmental phases of avian embryos when sub- 
mitted to hyper-normal than sub-normal temperatures. With such pre- 
liminary remarks I may now briefly indicate the changes which I observed 
during the first six hours or thereabout in a clutch of naturally incubated 
eggs of the house-sparrow (Passer domesticus). The site of the nest was so 
conveniently near my laboratory that not more than about two minutes 
was required to transport an egg for examination. On the fifth egg being 
laid the mother bird began to sit. One egg was then removed and the 
germinal disc, circular in outline, was seen to be 2°5 millimetres in diameter. 
The pellucid area was very distinct and contrasted strongly with the external 
opaque area, both being almost circular. The second egg was removed after 
the bird had been sitting for about forty-five minutes. Examination showed 
a well-defined primitive streak with little of the embryonic shield remaining. 
Viewed from above the primitive streak was seen scored along its entire 
length. The posterior end of the streak falls short of the posterior border 
of the area pellucida. The diameter of the disc measured four millimetres. 
The third egg was allowed to incubate for fifteen minutes longer. The two 
zones were slightly oval, though the entire disc remained circular. The 
posterior part of the primitive streak was thickened into a distinct round 
knob which reached to the posterior end of the zona pellucida. The diameter 
of the entire disc measured six millimetres. The mesoblast had invaded 
laterally the zona pellucida, and a faint trace of the neural groove was seen 
to commence in front of the primitive streak. The amniotic fold was also 
faintly discernible. 
The fourth egg was removed an hour later. On the disc was seen the 
dorsal opening of the neurenteric canal situated between the front of the 
primitive streak and the posterior end of the neural canal. The anterior 
half of the latter had not yet appeared. The entire disc had not increased 
to a larger size than the preceding one. 
Opportunity was not afforded me of removing the last egg until four 
more hours had elapsed. Examination then showed that the entire dise was 
no larger than the preceding ones, but the neural groove had appeared along 
its entire length and the amniotic fold was quite distinct. Comparing these 
