CHAPTER XIV. 



GERM-niSTORY OF THE AMPHIOXUS AND OF THE 



ASCIDIAN. 



Relationship of the Vertebrates and Invertebrates. — Fertilization of the 

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

 Spherical Germ-membj'ane Vesicle (Blastula). — From this the Intes- 

 tinal Larva, or Gastrula, originates by Inversion. — The Gastrula 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 Large Gi!l-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. 

 — Retrogressive Development of the same. — The Tail with the Notochord 

 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 

 as that animal, which among all existing creatm-es is the one alone capable 



