656 MODE OF GROWTH OP THE LOWER VERTEBRATES. 



digits and limbs ; as well as the formation of the primitive skull, 

 with the development of the parts of the face, and the formation 

 of the anus. 



The cranium, from the researches of Rathke, Parker and others, 

 is formed from the middle germ-layer, and in the fourth day is 

 simply membranous ; after that time the tissue composing it be- 

 comes cartilage. After the fourth day the primitive skull consists 

 of two portions, i.e., a sheet of cartilage ensheathing the noto- 

 chord from its anterior end to the first vertebra. " This sheet of 

 cartilage forms an iinsegmented continuation of the vertebral bod- 

 axial skeleton, in which the segmentation has become obliterated ; 

 and as such is equivalent not to one, but to a (hitherto not cer- 

 tainly determined) number of vertebne." (Foster and Balfour. 

 For the farther changes in the development of the skull the reader 

 is referred to Parker's memoir on the Development of the Skull 

 of the Common Fowl, or the excellent, illustrated abstract in 

 Foster and Balfour's "Elements.") 



Not until the sixth day are distinct bird-characters developed. 

 Hitherto it would be almost impossible to distinguish the embryo 

 from a reptile or mammal. During the sixth and seventh day the 

 wing and foot assume a bird form, the crop and intestinal cceca 

 make their appearance, "the stomach takes the form of a gizzard, 

 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 onwards the embryo successively puts on 



genus, species and variety." By the ninth or tenth day the 

 feathers originate in sacs in the skin, these sacs by the eleventh 

 day appearing to the naked eye as feathers, the sacs however re- 

 maining closed as late as the nineteenth day, though many are an 

 inch in length. ^ 'Id 



"By the thirteenth day the cartilaginous skeleton is completed, 

 and the various muscles of the body can be made out with toler- 

 able clearness. Ossification begins, according to Von Baer, on the 



carpal bones of the hind-limb, and in the scapula. On the eleventh 

 or twelfth day a multitude of points of ossification make their ap- 

 pearance in the limbs, in the scapular and pelvic arches, in the 



