APPEXDIX, 395 



basket (respiratory cavity) ; (7) cavity of the body ; (m) muscles ; 

 (n) testicle (in tbe ascidia united witb the ovary into a herma- 

 phrodite gland) ; (o) amis ; (p) genital orifice ; (q) well-developed 

 embryos in the body cavity of the ascidia ; (r) rays of the 

 dorsal fin of the amphioxus ; (s) tail-fin of the amphioxus ; (w) 

 roots of the ascidia. 



Plate XII. shows the Ontogenesis, or the individual development 

 of the Ascidia (A) and the Amphioxus (B) in five different 

 stages (1-5). Fig. 1 is the Qgg, a simple cell like the egg of 

 man and all other animals (Fig. A 1 the egg of the ascidia, Fig. 

 B 1 the egg of the amphioxus). The actual cell-substance, or 

 the protoplasm of the egg-cell (?), the so-called yolk, is sur- 

 rounded by a covering (cell-membrane, or yolk-membrane), 

 and encloses a globular cell-kernel, or nucleus (y), the latter, 

 again, contains a kernel-body, or nucleolus (j:c) ; when the egg 

 begins to develop, the egg-ceU first subdivides into two cells. 

 By another sub-division there arise four cells (Fig. A2, B 2), and 

 out of these, by repeated sub-division, eight cells (vol. i. p. 190, 

 Fig. 4 0, D). By fluid gathering in the interior these form a 

 globular bladder bounded by a layer of cells. On one spot of its 

 surface the bladder is turned inwards in the form of a pocket (Fig. 

 A 4i, B 4<). This depression is the beginning of the intestine, 

 the ca\-ity (cZ 1) of which opens externally by the provisional 

 larval-mouth (cZ 4). The body- wall, which is at the same time 

 the stomach-wall, now consists of two layers of cells — the 

 germ-layers. The globular larva (Gastrula), now grows in 

 length. Fig. A 5 represents the larva of the ascidia. Fig. B 5 

 that of the amphioxus, as seen from the left side in a somewhat 

 more advanced state of development. The orifice of the intestine 

 {d 1) has closed. The dorsal side of the intestine {d 2) is con- 

 cave, the ventral side {d 3) convex. Above the intestinal tube, 

 on its dorsal side, the neural tube, the beginning of the spinal 

 marrow, is being formed, its cavity still opens externally in front 

 (g 2). Between the spinal marrow and the intestine has arisen 

 the spinal rod, or chorda dorsalis (Notochord) {c), the axis of the 

 inner skeleton. In the larva of the ascidia this rod (c) proceeds 



