82 METAMORPHOSIS. 



and the dorsal fin becomes more prominent, and is divided into two 

 parts. 



Besides these obvious external changes very great modifications 

 are effected in almost all the organs, which may be very briefly enu- 

 merated. 



1. Very profound changes take place in the skeleton. An 

 elaborate system of cartilages is developed in connection with the 

 mouth; the cranium itself undergoes important modifications; and 

 neural arches become formed. 



2. Considerable changes are effected in the gill pouches, and, 

 according to Schneider, whose statements must however be received 

 with some caution, the branchial sack becomes detached posteriorly 

 from the oesophagus, the oesophagus then sends forwards a prolong- 

 ation above the branchial sack which is at first solid. This prolong- 

 ation forms the anterior part of the oesophagus of the adult, and joins 



I the primitive oral cavity at the velum. The so-called bronchus of 

 I the adult is thus the whole branchial region of the Ammocoetes, and 



the anterior part of the oesophagus of the adult is an entirely new 



formation. 



3. The posterior part of the alimentary tract of the Ammocoetes 

 ' undergoes partial atrophy. The gall-bladder of the liver is absorbed ; 



and the liver itself ceases to communicate wdth the intestine. 

 ' 4. The eye undergoes important changes in that it travels to 

 the surface, and acquires all the characters of the normal vertebrate 

 eye. 



5. The brain becomes relatively larger but more compact, and 

 the optic lobes (corpora bigemina) become more distinct. 



6. The pericardial cavity becomes completely separated from the 

 body cavity, and a distinct pericardium is formed. 



7. The mesonephros of the larva disappears, and a fresh posterior 

 part is formed. 



Myxine. The ovum of Myxine when ready to be laid is inclosed, 

 as shewn by Allen Thomson \ in an oval horny shell in many respects 

 similar to that of Elasmobranchii ; from its ends there project a 

 number of trumpet- shaped tubular processes, which no doubt serve to 

 attach it to marine objects. No observations have been made on the 

 development. 



BiBLIOGKAPHY. 



(77) E. Calberla. "Der Befruclitungsvorgang beim Petromyzon Planeri." 

 Zeit.f. wiss. Zool. Vol. xxx. 1877. 



(78) E. Calberla. "Ueb. d. Entwicklung d. Medullan-ohres u.d. Chorda dorsalis 

 d. Teleostier u. d. Petromyzonten. " Morpholog. Jahrbuch, Vol. iii. 1877. 



(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtung am Ei d. Neun- 

 augen. Konigsberg, 1878. 



(80) Aug. Muller. "Ueber die Entwicklung d. Neunaugen." Miiller's 4rc7nu, 

 1856. 



^ Cyclopcedia of Atrnt. and Phys. Article ' Ovum. ' 



