DEVELOPMENT 



The development has been fully worked out in Petromyzon planeri.* 

 The egg is small, about 1 mm. in diameter. It is enclosed in a membrane 

 formed of an inner perforated and an outer structureless layer. Outside 

 there is a mucous envelope which causes the egg to adhere to foreign objects. 

 The male adheres to the female during oviposition and the ova are depos- 

 ited in a hole previously made and subsequently covered up, the fish moving 

 stones for this purpose by means of their suctorial mouths. The eggs are 

 laid in April and May. Segmentation is unequal but complete ; the gastrula 

 is formed by a combination of invagination and epibole, and the blasto- 

 pore persists as the anus. The central nervous system is formed by a solid 

 keel-like ectodermal ingrowth, in which the central canal arises by excava- 

 tion, and there is a solid cord of cells connecting the hind end of it with the 

 dorsal hypoblast. The pituitary body is formed from an invagination of 

 the ectoderm and subsequently becomes connected with the nasal pit r 

 with which its proximal part remains in communication throughout life,, 

 as the naso -palatine canal or pituitary pouch. 



The young are hatched as larvae which soon become Ammocoetes. These 

 live for three or four years, developing and increasing in size until they 

 become as large as or larger than the adult. They then undergo a sudden 

 (in three or four days) metamorphosis (from August to January) and 

 become adult (Fig. 48). The adult possesses ripe generative organs and 

 spawns in April-May. After spawning the lamprey (in the case of P. ftuvi- 

 atilis at least) dies. 



Ammocoetes was formerly regarded as a distinct genus of 

 animal, and separate species were distinguished. The fact that 

 it becomes metamorphosed into Petromyzon was discovered two 

 hundred years ago by L. Baldner, a fisherman of Strasburg, 

 and rediscovered by Aug. Miiller.t 



In Ammocoetes the buccal cavity is without the annular lip r 

 but possesses a semicircular upper lip (Fig. 59), and a small 

 separate lower lip. There are no teeth, but several fringed 

 barbels surround the mouth. The eyes are imperfect and 

 hidden beneath the skin. The gill openings are placed in a 

 groove on each side. The median fin extends all along the back, 

 as a continuous structure. The branchial pouches open into 

 the pharynx directly, and there is no suboesophageal tube or 

 bronchus distinct from the pharynx. In Ammocoetes there is 

 a gall bladder and bile duct, which opens into the intestine. 

 In the lamprey both these structures are absorbed, and the 

 intestine itself undergoes partial atrophy. The eye in Ammo- 



* F. M. Balfour, A Treatise on Comparative Embryology, vol. ii. 1881 

 (with literature to date). A. E. Shipley, Q. J. M. S., 27, 1887. C. Kupffer, 

 Arch. mic. Anat., 35, 1890. P. Bujor, " Metamorphose de 1' Ammocoetes 

 branchialis," Rev. Biol. Nord. France, iv. 1891, p. 41. 



t Miiller's Arch., 1856, p. 325. 



