ORIGIN OF MIDDLE GERM-LAYERS. 237 



Much recent research by Kowalevsky, Ray-Lankester, 

 Van Beneden, and others has justified this "Four-layer 

 Theory " of Baer. For instance, it can be plainly shown 

 that in the Earth-worm (Fig. 50), in the Amphioxus (Fig. 

 51), and in some other animals each of the two primary 

 germ-layers parts into two secondary germ-layers; the 

 skin, or outer-layer parts into the skin-sensory layer (hs), 

 and the skin-fibrous layer (km) : similarly the intestinal 

 or inner layer separates into the intestinal-fibrous layer 

 (df), and the intestinal-glandular layer (dd). The body- 

 cavity, or coeloma (c), forms between the two fibrous layers. 



Contrary to this view, most recent observers assume 

 that the two middle layers proceed from plane-division of 

 a single, middle germ-layer (mesoderma). According to 

 this, a third originates between the two primary layers, 

 and by a secondary process of fission splits into two layers 

 along the plane of its surface. Some observers, however, 

 as certainly derive this third layer from the lower primary 

 layer, as do the others from the upper primary layer. It 

 is exactly this suspicious circumstance, together with many 

 other grounds (based especially on Comparative Anatomy) 

 that lead us to the conjecture, which I believe to be correct, 

 that neither party is right, but that the outer middle 

 layer rather proceeds from the animal, the inner middle 

 layer from the vegetative germ-layer. It is true, as we 

 shall presently find, that only a single middle layer 

 (Remak's " motor-germinative germ-layer ") usually arises 

 between the two primary germ-layers of mammals, and 

 that by the fission of this, the two different middle layers, 

 the skin-fibrous layer and the intestinal-fibrous layer, 

 originate only secondarily. There are, however, strong 



