THE SIDE-LAYEKS. 303 



On both sides of this rise the two parallel dorsal 

 swellings, or spinal swellings (m). At the same time the 

 central notochord, or chorda dorsalis (Fig. 90 cfi), separates 

 entirely and definitely from the two lateral portions of the 

 mesoderm. These we will henceforth regard as side-layers 

 (sp) in reference to the axial chord. They are usually 

 called side-plates. In the middle of each of these side- 

 layers a horizontal fissure appears, where the upper or outer 

 skin-fibrous layer separates from the lower or inner intestinal- 

 fibrous layer. This fissure (Fig. 90 uwJi) is very significant, 

 for it represents the first rudiment of the future body- 

 cavity (cceloma). (Of. Plate IV. Fig. 2, c and 3, c.) 



In speaking of these side-layers, which are usually 

 called " side-plates," I would say a word or two about 

 those figurative expressions "layers" and "plates," which 

 have been universally employed since Baer's time. The 

 " layers " (laminae), as well as the " plates " (lamellae), are 

 leaf-like or plate-shaped bodies originally consisting of a 

 single homogeneous cellular stratum, or of several lying one 

 above the other, and constituting the first basis of the 

 organic systems and of the organs of the body. But the 

 language of Ontogeny distinguishes considerably between 

 a layer, or leaf (Lamina), and a plate (lamella). The first 

 and oldest cell-layers of the germ, which overspread the 

 whole germ, and form the first basis of whole organ-systems, 

 are layers, or leaves (laminae). On the other hand, the 

 term plates (lamellce) is applied to separate portions of the 

 layers, or leaves, and to the cellular strata produced from 

 the latter, which only belong to a part of the germ and 

 serve to form single organs of variable size. 



Of course this distinction is by no means sharply 



