104 



DE VEL OPMENT. 



77). The first trace of the embryo appears as a faint streak at the posterior 

 end of the area germinativa, called the primitive trace. After the formation of 

 the primitive trace, but previous to the appearance of the next parts of the 

 embryo, presently to be described viz. the laminae dorsales and the notochord 

 the blastodermic membrane consists of only two layers, the epiblast and hypo- 

 blast, but after the formation of these structures 

 a third layer makes its appearance. This is the 

 mesoblast, and is situated between the other two 

 (Fig. 78). The epiblast of the germinal disk is 

 formed of the most superficial layers of the inner 

 cells which were exposed by the disappearance 

 of the outer cells, which originally covered them, 

 the remaining epiblast of the blastodermic vesi- 

 cle being probably the persistent outer cells, 

 while the hypoblast is formed by the rest of the 

 inner cells. In the region of the primitive trace 

 the epiblast and hypoblast fuse together, and 

 from the sides of this line of fusion cells grow 

 out laterally into the space between the epiblast 

 and hypoblast to form the mesoblast, a further 

 formation of this layer also taking place at the 

 margin of the germinal disk. The blastodermic 

 membrane thus comes to consist of three layers : The external, which used to be 

 called the serous layer, but is now more commonly termed the epiblast, or ecto- 



FIG. 77. Ovum with the germinal area, 

 seen in profile to show the division of the 

 blastodermic membrane. 1. Vitelline 

 membrane. 2. Blastoderm. 3. Germinal 

 area. 4. Place where the blastoderm is 

 just divided into its two layers. 



FIG. 78. Section across the anterior part of the medullary groove of an early embryo of the guinea pig. (By 

 Schafer. From Quain's Anatomy, 1890.) ep. Folds of epiblast rising up on either side of the middle line, and 

 thus bounding the medullary groove, m.g. Middle of medullary groove, hy. Hypoblast, which is in contact with 

 the medullary epiblast at the middle of the groove, but is elsewhere separated from it by mesoblast, m, which 

 has burrowed forward between the two primary layers. A cleft is seen in the mesoblast on either side ; this is 

 the commencement of the anterior part of the body-cavity. 



derm : the internal, the mucous layer, the hypoblast, or entoderm ; and the middle, 

 which is now usually called the mesoblast or mesoderm, but which was formerly 

 named the " vascular layer." 



The epiblast is mainly concerned in the formation of the external cuticle and 

 the whole of the nervous system. It consists of cells of an epithelial character ; 

 that is to say, cells of an irregular columnar shape, forming, for the most part, a 

 single stratum, but becoming more numerous and flattened at the germinal disk. 

 The epidermis of the body and all the involutions of the epidermis in the ducts of 

 superficial glands, as the mammae, as well as the brain, the spinal cord, the nerves, 

 and the portions of the nose, eye, and ear, which are directly formed from the 

 brain, are developed from it. The external layer of the amnion is also formed 

 from the epiblast, and probably also a portion of the chorion. 



The hypoblast is mainly concerned in forming the internal epithelium viz. 

 that of the whole alimentary passages except the mouth and a small portion of the 

 rectum near the anus (which are formed by involutions of the epiblast) ; that of 

 the respiratory tract, which is originally an offset from the alimentary canal ; and 

 the epithelium of all the glandular organs which open into the intestinal tract. 

 The hypoblast forms also the deeper layer of the umbilical vesicle and allantois. 



