498 MANUAL OF ELEMENTARY ZOOLOGY 



the coelomic epithelium. The greater part of the ccelom 



becomes the abdominal (pleuroperitoneal) 



Tissues? 9 *' cavity, surrounding the gut on all sides except in 



the mid-dorsal line, where the mesentery is left. 



A forward ventral prolongation of the ccelom becomes the 



pericardial cavity. The muscles of the gut arise from the 



splanchnic layer, the body muscles from the mesoblastic 



somites, which give rise to myotomes, though the regular 



arrangement of these is lost in the adult. The bulk of the 



skeleton is at first laid down in cartilage, which in places 



becomes converted into bone and in places is reinforced by 



membrane bones, as has been explained in the chapter on 



Lc the skeleton of the adult. The first 



rudiment of the cranium has the 



form of a pair of curved longitudi- 



pq- nal bars, the trabecules, lying below 



the brain, and joined behind to a 



t.,. pair of parachordal plates at the 



sides of the front end of the noto- 



chord, which projects into the floor 



Fig. 378.-A diagram of of the tadpole's skull as it does into 



the rudiment of the that of the dogfish. Between the 



skull in a tadpole. trabecular is at first a space or 



au.c, Auditory capsule; i.e., " fossa," in which lies the pituitary 



S-CTtui^S"; bod y- These structures fuse with 



$.j>.q., paiato-pterygo- one another and with the cartila- 



SoS; ^.Vtratc u r a " ginous nasal and auditory capsules, 



and upgrowths from them form the 



sides and eventually the roof of the cranium. The pituitary 



opening presently closes. The continuous paiato-pterygo- 



quadrate bar of cartilage, which forms a part of the cartilage of 



the mandibular arch, is at first the only skeleton of the upper 



jaw. The hyoid apparatus of the adult is the remains of the 



skeleton of the hyoid and branchial arches of the tadpole. 



The heart appears some time before hatching. It is at 

 bi odv ei ^ rst a stra 'ght tube, which arises below the 

 " pharynx. Subsequently the tube is thrown 

 into an S shape and becomes divided by partitions into the 

 several chambers. The endothelium or pavement epithe- 

 lium which lines the heart arises by the rearrangement of 

 some scattered cells which lie between the splanchnic layer 



