684 COMPARATIVE ANATOMY 



platysma. The integumentary muscle of the neck. 



pleura. The serous membrane which lines the chest and covers the lungs. 

 plexus. A network of nerves or blood vessels. 

 plica. A fold or pleat. 



polymerization. The chemical synthesis of two or more molecules to form a new com- 

 pound without the production of a secondary compound. 

 poljrmorphonuclear leucocyte. A white blood corpuscle which has a nucleus with 



irregular constrictions. 

 polyp. The sessile or attached stage of a coelenterate. 



polyphyletic theory. The theory that the various blood cells have had a multiple origin. 

 polyphyodont. Having more than two sets of teeth. 

 pons. A bridge of fibers below the cerebellum which connects cerebrum, cerebellum 



and medulla oblongata. 

 postanal gut. That part of the embryonic digestive tract posterior to the anus, 

 postfrontal. A roofing bone of the skull posterior to the frontal. 

 posttrematic branch. That division of a cranial nerve which forks behind a gill-slit. 

 precoracoid. The anterior of the two ventral elements of the pectoral girdle, 

 pregnancy. The condition of being with child. Gestation. 

 premandibular cavity. The second somite which forms the superior oblique eye muscle 



and — it is asserted — a part of the external rectus. 

 premolar. A bicuspid tooth. 



preoral gut. That part of the embryonic intestine which is anterior to the mouth, 

 prepuce. The skin fold which covers the glans penis or the clitoris. 

 pretrematic branch. The nerve branch which forks in front of a gill-slit. 

 primitive duct. The pronephric duct. 



primitive streak. The elongated and closed blastopore of amniote embr^'os. 

 process theory. The theory that nerve fibers (axons) develop as processes of neuroblast 



cells. 

 proctodeum. That portion of the hindgut which is lined by ectoderm, 

 progestin. A luteal hormone which affects the endometrium of the uterus, 

 progynon. A proprietary name for the female sex hormone. 

 pronation. The act of turning the palm of the hand downwards. 

 pronephroi. The primitive vertebrate kidneys. 



proprioceptors. The mechanisms for receiving stimuli from within the body, 

 propterygium. The anterior basal element of the fish extremity. 

 prosencephalon. The anterior division of the embryonic brain which forms the cerebral 



hemispheres. 

 prosimian. Pertaining to primitive apes, 

 prostate gland. A muscular gland which surrounds the urethra where it leaves the 



bladder. 

 prostomium. The preoral lobe of Annelids. 

 proterostomia. Those animals in which the blastopore becomes the mouth or lies near 



the mouth. 

 proteose. A soluble protein formed by hydrolytic cleavage of a protein. 

 protochordate. The primitive chordates which do not acquire a vertebral column. 

 protocone. The inner cusp of an upper molar. 

 protoconid. The outer cusp of a lower molar. 

 protonephridia. Primitive excretory tubules without nephrostomes and with soleno- 



cytes. 

 prototype. The original form or t>'pe from which others evolve. 

 proximal. Towards the body. 

 pseudocoelom. A false body cavity not lined by peritoneum. 



