236 



THE EVOLUTION OF MAN. 



of the two middle layers, which I, in opposition to most 

 other authors, still hold to be correct. That is to say, he 

 derived each middle layer separately from a primary germ- 

 layer (by fission), and said, that the outer or animal germ- 



FIG. 50. Transverse section through the embryo of an Earth-worm : hs, 

 skin-sensory layer ; hm, skin-fibrous layer ; df, intestinal-fibrous layer ; dd, 

 intestinal-glandular layer ; a, intestinal cavity ; c, body-cavity, or Cosloma ; 

 n, nerve-centres ; it, primitive kidneys. 



FIG. 51. Corresponding section of the larva of Amphioxns (after 

 Kowalevsky). The letters indicate the same parts as in Fig. 50. 



layer separates into two strata, a skin-stratum and a flesh- 

 stratum ; similarly the inner or vegetative germ-layer 

 separates into two strata; the vascular stratum and the 

 mucous stratum. In the following table this view of Baer, 

 which I believe to be right in regard to the phylogenetic 

 origin of the middle layers, is compared with the newer 

 nomenclature, which is now in vogue : 



A. The two primary germ-layers. B. The four secondary germ-layers. 



I. The outer or animal germ- 



layer (the skin-layer, or 

 exoderm). 



II. The inner or vegetative 



germ-layer (the intesti- 

 nal layer, or entoderm). 



1. Skin-sensory layer (skin-stratum, Baer). 



2. Skin-fibrous layer (flesh-stratum, Baer). 



3. Intestinal-fibrous layer (vascular stra 



turn, Baer). 



4. Intestinal-glandular layer (mucous stra- 



tum, Baer). 



