Tabular Review. 59 



begins in the gastreads or sac shaped animals as a simple outside skin 

 the exoderm. The intestinal system begins with the same animals 

 as the entoderm, a simple inside lining of the outer sac. In the worms, 

 however, the outer skin has developed into a double skin, and the inner 

 skin or lining has also formed itself into two. In the mammals the 

 outer skin develops hairs and glands. In the chorda animals (ascidians 

 &c.) the respiratory and digestive apparatus are developed from the en- 

 toderm. So, also, the nervous system first appears in the primitive 

 worms as a throat ganglion. In the chorda animals it becomes a sim- 

 ple medullary tube, while in the monorhina, such as lampreys, hag fish, 

 &c. , it expands to a brain and spinal marrow. The blood and lymph 

 vascular system first occurs as a simple space or coelom between the in- 

 ner skin and the outer skin in the scolecida or soft worms. In the 

 worms the dorsal and ventral blood vessels are differentiated in this 

 body cavity one above and the other below the primitive intestinal 

 tube, &c. , &c. 



Table X shows the origin of the chief organ systems as they occur in 

 the human body, which is, of course, typical of the mammalian class. 

 The column on the right indicates the particular layers of the germ 

 shield involved in the production of the organ opposite. Thus it can 

 be seen at a glance that the "skin sensory layer" forms the epidermis, 

 and is involved in the development of the brain and spinal marrow and 

 brain nerves, all the organs of sense, touch, taste, smell, sight, and 

 hearing ; the genitals and probably the kidneys. The skin fibrous layer, 

 or second membrane, is concerned in the leather skin, brain, spinal and 

 intestinal nerves, the sense organs, the muscles and skeleton, the blood 

 vessels, kidneys and reproductive organs. The third or intestinal 

 fibrous la}'er is concerned in the formation of the sympathetic nerves, 

 the digestive and respiratory systems, the vascular system and heart, 

 the urinary bladder and the ovary. The fourth or intestinal glandular 

 layer is concerned in the production of the intestinal tube and its appur- 

 tenances the digestive and respiratory organs, the urinary bladder and 

 probably the ovary. 



