45O THE EVOLUTION OF MAN. 



systems of the body, it is in the muscle-system that arti- 

 culation first appears. 120 



While these characteristic differentiations are taking 

 place in the two lamellae of the animal germ-layer while 

 the medullary tube and the outer skin (epidermis) are 

 separating from the skin-sensory layer, and the notochord 

 and the muscle-plates from the skin-fibrous layer, equally 

 important processes, characteristic of the vertebrate type, 

 are taking place in the vegetative germ-layer. The inner 

 lamella of this the intestinal-glandular layer undergoes 

 but few modifications; it produces only the internal cell- 

 coating, or epithelium of the intestinal tube (d). But the 

 outer lamella, the intestinal-fibrous layer, produces both 

 the muscular covering of the intestine and the blood- 

 vessels. Probably simultaneously, two main vessels ori- 

 ginate from this layer : an upper, or dorsal vessel, corre- 

 sponding to the aorta, situate between the intestine and the 

 chorda dorsalis (Figs. 13, t, 15, f) ; and a lower, or ventral 

 vessel, answering to the heart and the intestinal vein, on 

 the lower edge of the intestine, and between it and the 

 ventral skin (Figs. 13, v, 15, v). Moreover, at this time 

 the gills, or respiratory organs, also develop in the anterior 

 portion of the intestinal canal. The whole anterior or 

 respiratory section of the intestine changes into a gill-body, 

 which is pierced by numerous openings, so that it resembles 

 a lattice-work, as in Ascidia. The cause of this is that the 

 foremost portion of the intestinal wall adheres in places 

 to the external skin, and that, at these points of adhesion, 

 openings form in the wall and extend from outside into 

 the intestine. At first these gill-openings are but very few, 

 but soon they are numerous, appearing first in one row, 



