308 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



there being hardly any food-yolk, and its wall, from the first, 

 is partly formed of ectoderm, the other layers subsequently ex- 

 tending into it. In the Chick all the layers grow round to 

 invest the yolk, which is at first covered only by the vitelline 

 membrane. The embryo is folded off as described previously for 

 the Chick, and it is only at a comparatively late date that special 

 characteristics are developed. It must, however, be remembered 

 that though four visceral clefts (Fig. 94) are present, the last 

 one is not bounded behind by an arch. 



3. Fate of the Germinal Layers. The three layers give rise 

 to the same parts as in the Chick (p. 263), but a few remarks 

 are necessary concerning the mesodermic structures. 



Cartilaginous bars are developed in the first three visceral 

 arches, which develop as follows : 



I. Mandibular. Incus, Malleus, and Meckel's cartilage of either side, 

 Eustachian cartilage, Pterygoid, and Palatine. 



TT ir-v7 ( * use ventrally to form 1 anterior 



\ body of hyoid, / cornua ; stapes. 



- Branchial 1, { P^ 



The aortic arches develop in a similar way to those of the 

 Chick (p. 265), but the left th arch becomes the aorta, and the 

 right Mh the right subclavian, while the rigM 5th arch disappears, 

 and the left 5th forms the pulmonary artery. 



The anterior cardinals become the external jugulars, and the 

 Cuvierian ducts the precavals. 



The history of the excretory organs is much as in the Chick 

 (p. 265), but there is no trace of a pronephros. 



4. Embryonic Appendages (Fig. 95). These are modified in 

 accordance with the important fact, that while in the Chick 

 development mainly goes on outside the parent's body, at the 

 expense of the bulky food-yolk, the embryo Rabbit develops 

 mainly within the body of the mother, and is born " alive." In 

 other words, the Bird is oviparous, the Mammal viviparous. 



It must be borne in mind that the blastocyst is closely invested 

 by the remains of the zona radiata of the ovum. 



The yolk-sac (umbilical vesicle) is formed by that part of the 

 blastocyst which is outside the embryonic area. From the first 

 its wall is formed within the zona radiata by ectoderm, beneath 



