DEVELOPMENT OF ANIMALS. 647 



layer are destined to arise the skin and wall of the body 

 with the nervous system ; from the second (mesoderm, in 

 the embryo called the mesoblast) are formed the heart and 

 the vascular system, as well as the stomach and intestines. 



The middle layer now thickens, causing the mark known 

 as the " primitive streak," along the middle of which runs 

 the " primitive groove." The notochord now appears and 

 the primitive vertebrae (protovertebrce, Fig. 540). The am- 

 nion arises as a membrane, splitting off from the outer germ- 

 layer of the embryo, and finally forms a cavity which is 

 filled with a fluid. About this time the allantois arises as 

 an offshoot of the alimentary canal, budding out at the 

 hinder end of the embryo, and finally curving over the em- 

 bryo, serving as a foetal respiratory membrane. 



Second Day. The three portions or vesicles of the brain 

 now appear (Fig. 540), as well as the alimentary tract and 

 heart, both arising in the head-fold or enlargement (Fig. 540, 

 a to c), and soon the blood-vessels arise as channels in which 

 blood -corpuscles appear, originating as amoeba-like cells 

 separating from the cellular mass of the mesoderm. Dur- 

 ing the second day also the eyes and ears begin their devel- 

 opment, being at first simply folds or inpushings of the 

 outer germ-layer. 



Third Day. This is one of the most eventful days, as im- 

 portant steps in the elaboration of the different organs are 

 taken ; the different parts of the brain, of the alimentary 

 tract and its appendages being sketched out, and the rudi- 

 ments of the lungs, the liver, pancreas, nose, and different 

 parts of the eye and ear appearing. On the fourth day the 

 wings and legs grow out, appearing first as flattened buds. 

 The notochord, which is indicated by the second day, by the 

 sixth begins to diminish in size, disappearing by the time 

 the chick is hatched, while by the twelfth day the deposition 

 of bone in the bodies of the vertebrae commences. Between 

 the eightieth and one hundredth hour the internal differences 

 in the sexes appear, the testes beginning to arise on the 

 sixth day. 



fifth Day. The limbs have by this time developed so as 

 to show the knee- and elbow-joints, as well as the cartilages 



