Generation and Development. 383 



time becomes pear-shaped, and corresponds to the direction 

 of the future embryo. 



At the posterior part of the germinal area the primitive 

 streak appears, and soon becomes a groove termed the 

 primitive groove. 



Leaving the ovum for the moment, we must describe the 

 changes which have occurred in the blastodermic layer 

 which was left on the inside of the vitelline membrane. It 

 is found that this layer divides at first into two, and then 

 into three layers. The outer, called the ectoderm, epiblast, 

 or serous layer, grows the central nervous system and 

 epidermal tissues, including hoof. The inner or lower 

 layer is termed the endoblast or hypoblast, and grows the 

 intestinal canal and the glands opening into it. The 

 middle layer, or mesoblast, forms the bloodvessels, and 

 constitutes a part termed the area vasculosa : it also grows 

 the muscles and skeleton, generative and excretory organs. 

 Returning to the primitive groove, it is found that in 

 front of it is formed another groove, the medullary, which, 

 extending back and opening out, encloses the primitive 

 groove and causes it to disappear. The growth of the 

 medullary groove has given rise to a part termed the area 

 pellucida, surrounded by an opaque surface, termed the 

 area opaca. In the pellucid area is formed the embryo. 

 The medullary groove grows from the epiblast ; from either 

 side of the groove rises up a structure termed the lamina 

 dorsales, which, growing over the groove, meet, and thus 

 form a canal termed the neural canal ; in this is developed 

 the brain and spinal cord. 



Underneath the medullary groove is formed from the 

 mesoblast a cord termed the notochord, from and around 

 which are developed the vertebrae. From another portion 

 of the mesoblast is formed the pleuro-peritoneal cavity. 

 Two important structures appear at this time, developing 

 from one or other of the layers into which we have seen 

 the blastodermic membrane is divided. By the union of 

 certain layers of the mesoblast with the epiblast a structure 

 termed the somatopleure is formed, and from a layer of the 



