288 Cell as Basis of Organic Activity 



to form the hollow neural tube. This separates from the main portion 

 of the ectoderm, and the anterior portion enlarges to become the brain. 

 On either side, thickenings produce the roots of the spinal and cranial 

 nerves while other thickenings in the head region develop into the 

 sensory portions of the eye and ear. In addition to giving rise to the 

 nervous system and sense organs, the ectoderm by virtue of its position 

 develops into the outer portion of the skin and its derivatives. This 

 includes the epidermis and epidermal structures such as the hair, teeth, 

 glands, scales, etc. Two important endocrine glands are also derived 

 from the ectoderm : the pituitary and the medulla of the adrenal gland. 



Entoderm. — Since entoderm lines the embryonic gut, it develops 

 into the epithelial lining of the digestive tract with the exception of 

 small portions of the mouth and anus. It also lines the various out- 

 pocketings that develop from the tract. These include the thyroid, 

 thymus, liver, pancreas, the bladder, part of the middle ear, and the 

 respiratory tract. 



Mesoderm. — The mesoderm forms the greatest portion of the 

 organs of the body. In vertebrates at the time that the lateral out- 

 pocketings of the mesoderm arise, a strengthening rod, the nolochord, 

 is also pinched off. This notochord is the beginning of the axial skele- 

 ton ; in higher vertebrates, the notochord finally forms the pads between 

 the individual vertebrae. The lateral outpocketings of mesoderm grow 

 ventrally from their dorsal origin to meet below the archenteron. By 

 differential growth, the mesoderm next separates into three regions : a 

 dorsal epimere, a middle mesomere, and a ventral hypomere. 



The dorsal epimere becomes arranged in segmental blocks or 

 somites on either side of the neural tube. These are solid blocks of 

 tissue with the central cavity closed. Soon they become completely 

 separated from the remaining portion of the mesoderm and give rise 

 to part of the dermis and the voluntary muscles. 



The mesomere does not form somites and retains its close associa- 

 tion with the hypomere. The mesomere develops into the excretory 

 system, the tubes of which open into the hypomere. The cavity of 

 the mesomere remains as the cavities of the tubules of the kidneys. 



The hypomere also does not form somites. It retains its cavity as 

 the coelom, and the somatic and splanchnic layers form the lining peri- 

 toneum. Dorsally and ventrally where the two sides meet, the ventral 

 and dorsal mesenteries are formed. The splanchnic mesoderm gives 

 rise to the heart. 



