DEVELOPMENT OF ANIMALS. 649 
which precede the formation of the bones of the digits and 
limbs. The primitive skull also arises from the mesoderm. 
Until the sixth day it would be impossible to say whether 
the embryo was that of a bird, reptile, or mammal, but now 
the characters peculiar to birds appear. The wings and legs 
manifest their bird-like characters, the crop and intestinal 
ceca are indicated, ‘‘ the stomach takes the form of a giz- 
zard, and the nose begins to develop into a beak, while the 
incipient bones of the skull arrange themselves after the 
avian type. . . . From the eleventh day onward, the embryo 
successively puts on characters which are not only avian, 
but even distinctive of the genus, species, and variety ”’ 
(Balfour). By the ninth or tenth day the feathers originate 
in sacs in the skin, while the nails and scales begin to ap- 
pear on the thirteenth day, and at this time the various 
muscles of the body can be distinguished. Development is 
thus seen to be from the general to the special. from the 
simple to the complex ; the trunk is first indicated ; while 
the peripheral parts—i.e., the extremities, the digits, the 
skin, feathers or scales, or hair, whatever be the type of 
Vertebrate—are the last to be elaborated ; in other words, 
the characters of the branch, class, and order are the first 
to be evolved, those of the family, genus, and species the 
last. 
The development of the rabbit, guinea-pig, or any mam- 
mal, including even man, follows much the same order as 
in the chick, there being, however, a well-marked morula ; 
the differences are due to the fact that the embryo maminal 
d, yolk-skin ; @’, villi of the yolk-skin ; sh, serous membrane ; 62, villi of the serous 
Membrane ; ch, chorion (vascular layer of the allantois); chz, true villi of the chorion 
{arising from the projections of the chorion and the sac of the serous membrane); 
am, amnion ; ks, head-fold of the amnion ; sg, tail-fold of the amnion ; ah, cavity of 
the amnion ; as, sheath of the amnion for the navel-string ; a, the first beginning of 
the embryo arising from a thickening of the outer layer of the blastoderm a@/; m, 
thickening forming the germ in the middle layer of the blastoderm (m/), which at first 
only reached as far as the germinal disk, and afterward forms the vascular layer of 
the yolk-sac (@f) which connects with the intestino-muscular layer (darmfaserblatt); 
st, sinus terminalis ; dd, yen eee layer (darmdrusenblatt) arising out of a 
part of i, the inner layer of the blastoderm (afterward the epithelium of the yolk- 
sac); kh, cavity of the blastoderm, which afterward becomes (ds) the cavity of the 
yolk-sac ; dg, passage way of the yolk; ai, allantois; e, embryo; 7, original space 
between the amnion and chorion, filled with albuminous fluid ; 2, anterior body-wall 
in the region of the heart ; Ah, cavity of the heart without the heart itself. In Figs. 
+ and 3, fhe amnion is, for the sake of clearness, represented as situated too far away 
from the embryo; s0 also the rae of the heart is drawn too small and the embryo 
too large, since, except in Fig. 5, they are only drawn diagrammatically.—From Kol- 
liker’s ‘*‘ Entwickelungsgeschichte des Menschen und der hdheren Thiere.”” 
