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DEVELOPMENT OF THE CHICK EMBRYO. 639
embryo of the hen. The embryo doea not contain any gelatin-forming
substance until the tenth day, and from the fourteenth da}’ on it contains
a body which, when boiled with water, gives a BUDStance similar to chon-
drin. A body similar to mucin occurs in the embryo when about six
days old, but then disappears. The quantity of haemoglobin shows a
continual increase compared with the weight of the body. Liebermann
found that the relation of the haemoglobin to the body weight was 1:728
on the eleventh day and 1:421 on the twenty-first day.
By means of Berthelot’s thermometric methods Tangl x
has
determined the chemical energy present at the beginning and end of
the development of the embryo of the sparrow’s and hen’s eggs. The
difference was considered as work of development. He found that the
chemical energy necessary for the development of each gram of ripe hen’s
embryo (Plymouth) was equal to 0.805 Cal. This energy originated
chiefly from the fat. Of the total chemical energy utilized, about 70
per cent was used for the embryo and about 30 per cent remained in the
yolk. Of the utilized energy about two-thirds was used in the con-
struction of the embryo and about one-third transformed into other
forms of energy as work of development.
By their investigations on the development of the trout egg, Tangl
and Farkas 2
have found that the loss in weight of each egg which had
an average weight of 88 milligrams was 4.9 milligrams during the 42
days of incubation, of which 4.11 milligrams was water and 0.722 milli-
gram dry substance with 0.367 milligram C. The eggs lose no nitro-
gen and no fat. The fat content increases a little, and indeed, as these
authors believe, at the expense of the proteins. The chemical energy
used during development was 6.68 gram-calories.
The highly interesting investigations made by Loeb upon the fer-
tilization of the eggs of lower sea-animals will be discussed in this con-
nection. According to these experiments after the fertilization of the
egg by means of a sort of cytolysis small drops of a colloid substance
form on the surface of the egg. These drops enlarge in volume and
conglomerate to a continuous mass, while its surface hardens to a tight,
continuous membrane—the fertilization membrane. The process of
membrane formation is in fact the essential step in the fertilization.
Besides, by spermatozoa, the membrane formation is caused by different
actions. For many eggs all that is necessary is the artificial calling
forth of the processes for the membrane formation in order that the
egg shall develop to normal larvae (for example the eggs of the star
fish and of certain worms). In other cases, for example the sea-urchin,
Strongylocentrotus, a second action is necessary for the production of
1
Pfluger’s Arch., 93 ani 121. !
Ibid., 104.
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