218 ESSENTIALS OF ZOOLOGY 



large intestines develop from the posterior portion of this origi- 

 nally straight tube with relatively slight modification. The liver 

 appears as a tubular outgrowth from the duodenum as likewise does 

 the pancreas. 



The Respiratory System. — The trachea develops as a posterior ex- 

 tension of the pharynx, which is separate from the esophagus. It 

 begins as a groove, extends posteriorly, and soon bifurcates, forming 

 the bronchii from which the two lungs develop. This original struc- 

 ture, therefore, is endoderm, and the internal lining of the adult 

 trachea and lungs is the portion derived from this. Again, the 

 splanchnic mesoderm has contributed the muscle and connective tis- 

 sue outside of the lining. 



Nervous System and Sense Organs 



At the end of gastrulation the ectoderm has become a distinct 

 layer. The portion of it which will become the mid-dorsal area of 

 the body thickens to become the neural plate. This plate sinks, 

 forming a neural groove with a neural fold as each lateral boundary 

 of it. The groove deepens, and the folds grow to meet each other 

 over the cavity thus forming the neural tube. This tube is the fore- 

 runner of the spinal cord and brain. 



The Brain. — The anterior end becomes swollen and thickened for 

 the development of the brain. This brain structure divides by con- 

 strictions into the anterior forehrain (prosencephalon), midbrain 

 (mesencephalon), and the posterior hind brain (rhombencephalon). 

 Later, the forebrain divides into two regions, the anterior cerebral 

 hemispheres (telencephalon) and the posterior diencephalon. The 

 midbrain, or mesencephalon, remains undivided to become optic lobes 

 and related structures. The hind brain divides to furnish the cere- 

 bellum (metacephalon) from its anterior portion and the medulla ob- 

 longata (myelencephalon). During the time of this development the 

 brain becomes bent or flexed ventrally on itself so it becomes short- 

 ened and compact. 



Sense Organs. — These organs all develop from the ectoderm. The 

 organs of four of the principal senses develop from the peripheral 

 ectoderm, but those of the fifth, the sensory portions of the eyes, are 

 essentially processes of the ectoderm wall of the brain. 



The sensory portions of the eyes form as hollow evaginations (out- 

 growths) from the sides of the forebrain (diencephalon portion) 

 growing laterally toward the peripheral ectoderm of the head. 



