THREEFOLD SEPARATION OF THE MESODERM. 307 



accomplishes the excretory functions of the body, and se- 

 cretes the urine of the embryo. The primitive kidney 

 originally consists of an entirely simple, tubular, elongated 

 passage, a straight duct situated on each side of the ventral 

 aspect of the primitive vertebral cord, running from an 

 anterior to a posterior direction (Fig. 92, wag). It 

 apparently arises from the horn-plate, and at the side 

 of the medullary tube (spinal tube), in the space between 

 the primitive vertebral cord and the side-plates. Even 

 while the medullary tube is separating from the horn-layer, 

 the primitive kidney is visible in this gap. Some authors, 

 however, hold that the first rudiment of the primitive 

 kidney is not furnished by the skin-sensory layer, but by 

 the skin-fibrous layer. 



While the skin-sensory layer is thus splitting up into 

 the horn-plate, the spinal tube, and the primitive kidneys, 

 the mesoderm, or fibrous layer, also separates into three por- 

 tions, viz. : (1) the notochord in the central line of the germ- 

 shield (Fig. 92, ch) ; (2) the primitive vertebral bands on 

 each side of the notochord (uw) ; and (3) the side-layers which 

 separate from the exterior of primitive vertebral bands. 

 These side-layers still show the original separation of the 

 middle germ-layer into the outer skin-muscle layer (or skin- 

 fibrous layer, hpl), and the inner intestinal-muscle layer 

 (or intestinal-fibrous layer, df). The point of union of the 

 two fibrous layers is called the middle plate, or mesentery- 

 plate (mp). The narrow fissure (sp), or empty space 

 which arises between the two fibrous layers, is the first 

 rudiment of the body-cavity (coeloma), the great visceral 

 cavity, in which the heart, lungs, intestines, etc., are after- 

 wards situated. In Mammals this is separated, at a later 



