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 

 coeca 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 mammal 



d^ yolk-skin ; rf', villi of the yolk-skin ; sA, serous membrane ; sz, villi of the serous 

 membrane ; ck, 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 ; an, 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/ ; tyi, 

 thickening forming the germ in the middle layer of the blastoderm (m'), which at first 

 only reached as far as the germinal dislc, and afterward forms the vascular layer of 

 the yolk-sac (rf/0 which connects with the intestino-muscular layer (darmfaserblatt); 

 it, sinus terminalis ; dd, intestino-glandular 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 (lU) the cavity of the 

 yolk-sac ; dg, passage way of the yolk ; al, allantois ; e, embryo ; r, original space 

 between the amnion and chorion, filled with albuminous fluid ; vl, anterior body-wall 

 in the region of the heart ; hh, cavity of the heart without the lieart itself. In Figs, 

 i and 3, the amnion is, for the sake of clearness, represented as situated too far away 

 from the embryo ; so also the cavity 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 " Entwickelungegeschichte des Menschen und der hOhereu Thiere." 



