36 Dynamic Theory. 



It is on account of the striking divergence of the functions and out- 

 growths of these two germ layers that caused Baer to give them the 

 very appropriate titles of animal layer and vegetative layer, terms indi- 

 cative of their relationship to each other throughout the animal king- 

 dom; a relationship founded upon the necessity of every organized being, 

 of having domestic relations and foreign relations. 



In the four layer stage of the human embryo its value is equal to that 

 of a simple worm. A worm is, it is true, a finished animal, inasmuch 

 as the plates are closed around making a series of four tubes, one with- 

 in another, while the closing of the tubes in the human embryo is de- 

 ferred while additional developments and alterations are being made in 

 them. 



By the time the closing of the four human embryonic layers is accom- 

 plished, the embryo has passed through several worm stages, including 

 the simple one of the four plain and scarcely differentiated germ layers 

 which have their equivalents in the embiyos not only of worms but of 

 all the vertebrates; the slightly differentiated forms of the bloodless 

 worms, acoelomi, and the more complex organisms of the blood-bearing 

 worms or ccelomi. The distinction between the acoalomi and the ccelomi 

 is, that the former have not, and the latter have, a coelom or body cav- 

 ity. This body ca\ ity is formed, as already described, by the separa 

 tion of the outside skin (exoderm) from the inside (entodcrm). When the 

 two are formed into tubes, while the inside tube or sac fits neatly and 

 closely into the outside one, there is no room for stowing away the sys- 

 tem of blood vessels and other machinery for the elaboration and puri- 

 fying of the nourishment required for the higher grades of animals. 

 But in the advance of organic structure, as organs become necessary to as- 

 sist the primitive intestine in elaborating the food, they make room for 

 themselves by separating the ectoderm from the intestine, and into this 

 space they are crowded. All animals without this body cavity are 

 bloodless, that is, have no circulation of the blood. 



By the formation of the body cavity and the differentiation of the gill 

 arches from the front end of the intestine, and by the introduction into 

 it first of the rudimentary kidney ducts for the excretion of the waste 

 matters, and the development of the two blood tubes above and below 

 the intestinal tube, together with the rudimentary system of nerves, the 

 human embryo attains to the organic value of a higher grade worm, 

 such, for instance, as the acorn worm, Balanoglossus. 



In the acorn worm, and in all other animals with gills, the gills are 

 at first slits in the front end of the intestine, and also, later, in the out- 

 side skin or exoderm. The part of the intestine thus sot off to the use 

 of gills, is called the gill body, and between it and the hind end of the 

 intestine a constriction is developed, which becomes the oesophagus. 



