AGE OF THE VASCULAR SYSTEM. 357 



animal body consecutively originate in the embryo, approxi- 

 mately to infer the phylogenetic sequence, in which these 

 organs gradually developed, one after the other, in the 

 ancestral line of animals. In the " Gastrrea theory " I made 

 the first attempt to establish the phylogenetic significance 

 of the ontogenetic sequence of the organ-systems; but it 

 must be remarked that this sequence is not always iden- 

 tical in the higher animal tribes. In Vertebrates, and 

 therefore also in our own ancestral line, the organ-systems 

 may be ranged according to age, in something like the 

 following order : I. The skin- system (A) and the intestinal 

 system (B). II. The nerve (C) and muscular systems (D). 

 III. The kidney system (E). IV. The vascular system (F). 

 V The skeleton system (G). VI. The sexual system (//). 

 (Cf. Table XXXIX, p. 3G7.) 



In the first place, the gastrula proves that in all animals 

 with the exception of the Primitive Animals (Protozoa), — 

 therefore, in all Intestinal Animals (Metazoa), — two primary 

 organ-systems originally arose simultaneously and first ; 

 these were the skin-system (skin-covering) and the intes- 

 tinal system (stomach-pouch). The first is represented, in 

 its earliest and simplest form, by the skin-layer or exoderm, 

 the latter by the intestinal layer or entoderm of the Gastraea. 

 As we can ascribe the same origin, and, therefore, also the 

 same morphological significance, to these two primary germ- 

 layers in all Intestinal Animals, from the simplest Sponge 

 to Man, the homology of these two layers seems sufficient 

 proof of the above assumption. 



Immediately after the differentiation of the two primary 

 germ-layers, an inner or outer skeleton develops in many 

 lower animals (e.g., in Sponges, Corals, and other Plant 



