46 AMPHIOXUS. 



degeneration has, however, been brought forward; and the 

 theory of degeneration would leave altogether unexplained what is 

 after all the most important fact, namely, the resemblance so often 

 referred to above, and seen not in one organ only, but in almost 

 every part of its structure, between the adult Amphioxus and 

 the embryonic stages of development of the higher Vertebrates. 



3. General Account of the Development of Amphioxus. 



The development of Amphioxus has, as yet, been studied by a 

 very limited number of investigators ; and many points, especially 

 in the later stages, are still only imperfectly understood. 



Our actual knowledge is due in the first instance to Kowa- 

 levsky, who published in 1867 an account of observations made 

 by him at Naples in 1864. His descriptions, though brief, are 

 exceedingly precise and well illustrated, and deal with both the 

 earlier and later stages of development ; they were supplemented 

 by further papers in 1870 and 1876. 



In 1881, Hatschek published a detailed and admirably 

 illustrated account of the earlier stages of development, from the 

 laying of the eggs up to the formation of the mouth and first 

 gill-cleft. His observations were made near Messina, the 

 specimens being obtained from a small salt lake, communicating 

 with the sea by a narrow channel two or three hundred yards in 

 length. 



The later stages of development, and more especially the 

 mode of formation of the gill-clefts, the endostyle, and the atrial 

 cavity were described very fully by Mr. Willey and Professor 

 Lankester in 1890 and 1891, from observations on specimens 

 obtained by Mr. Willey from the same locality as Hatschek. 

 Quite recently, 1892, Boveri has described the mode of formation 

 of the reproductive organs. 



The spawning period, in the Mediterranean, begins early in 

 spring, towards the end of March, and continues throughout the 

 summer, up to September ; June being apparently the month of 

 greatest activity. The eggs are laid about sunset, usually 

 between 7 and 8 P.M. ; they are very small, 0*105 mm. in diameter, 

 a,nd consequently contain but little food-yolk. They are fertilised 

 at once, by spermatozoa shed over them by the male, and begin 

 to develop almost directly. The early stages are passed through 

 with great rapidity, and early on the following morning, about 



