CHAPTER XIV. 



GEEM-HISTORY OF THE AMPHIOXUS AND OF TUB 



ASCIDIAN. 



RolationsTiip of the Vertebrates and Invertebrates. — Fertilization of the 

 Amphioxus. — The Egg undergoes Total Cleavage, and changes into n 

 Spherical Germ-membrane Vesicle (Blastula) .— Fvoxn. this the Intes- 

 tinal Larva, or Gastrula, originates hy Inversion. — The Gask'ula of the 

 Amphioxus forms a Medullary Tube from a Dorsal Furrow, and 

 between this and the Intestinal Tube, a Notochord : on both Sides the 

 latter is a Series of Muscle-plates; the Metamera. — Fate of the 

 Four Secondary Germ-layers. — The Intestinal Canal divides into an 

 Anterior Gill-intestine, and a Posterior Stomach-intestine. — Blood- 

 vessels and an Intestinal-muscle Wall originate from the Intestinal, 

 fibrous Layer. — A Pair of Skin-folds (Gill-roofs) grow out from the 

 Side-wall of the Body, and, by Coalescence, form the Ventral Side of 

 the Lai'ge Gill-cavity. — The Ontogeny of the Ascidian is, at first, iden- 

 tical with that of the Amphioxus. — The same Gastrula is Developed, 

 which forms a Notochord between the Medullary and Intestinal Tubes. 

 — Eetrogressive Development of the same. — The Tail with the Xotochord 

 is shed. — The Ascidian attaches itself firmly, and envelopes itself in 

 its Cellulose Tunic. — Appendicularia, a Tunicate which remains through, 

 out Life in the Stage of the Larval Ascidian and retains the Tail-fin 

 with the Chorda (Chordonia). — General Comparison and Significance of 

 the Amphioxus and the .Ascidian, 



" In the formation of its most important organs, the Amphioxus remains 

 throughout life at that lowest stage of development, which all other Verte- 

 brates pass rapidly through during the earliest period of their embryonic 

 existence. We must therefore regard the Amphioxus with peculiar reverence 

 Jk8 that animal, which among all existing creatures is the one alone capable 



