EXPLAI^ATION OF PLATES X. AND XI. 



Plate X. — Germ-history op the Ascidian and of the Amphioxue. 

 (Principally according to Kowalevsky.) 



Fig. 1-6. —Germ-history of an Ascidian. 



Fig. 1. — A parent-cell {cytula) of an Ascidian. In the bright-coloured 

 protoplasm of the parent-cell lies eccentrically a bright spherical kernel 

 (nucleus), and in the latter a darker nucleolus. 



Fig. 2. — An Ascidian egg in the process of cleavage. The parent-cell 

 has divided by repeated bisection into four similar cells. 



Fig. 3. — Membraneous germ-vesicle of an Ascidian (Blastula). The cells 

 resulting from the cleavage of the egg form a spherical bladder filled with 

 fluid, the wall of which consists of a single layer of cells. (Cf. Fig. 22, F, G.) 



Fig. 4. — Gastrula of the Ascidian resulting from the blastula (Fig. 3) 

 by inversion (invagination). The wall of the primitive intestine (d), which 

 opens at o by the primitive mouth, consists of two layers of cells ; the inner 

 intestinal layer formed of larger cells, and the outer skin-layer, of smaller. 



Fig. 5. — Larva of the Ascidian swimming freely. Between the medullary 

 tube (m) and the intestinal tube {d) the notochord is inserted (ch), which 

 passes throughout the long rudder-like tail to its point. 



Fig. 6. — Transverse section through a larval Ascidian (Fig. 5), through 

 the posterior part of the trunk just in front of the beginning of the tail. 

 The section is just the same as that of the Amphioxus larva (Fig. 11, 12). 

 Between the medullary tube (?n) and the intestinal tube (d) lies the noto- 

 chord (ch) ; on both sides are the lateral muscles of the trunk (r). 



Fig. 7-13. — Germ-history of the Amphioxus. 



Fig. 7. — Parent-cell (cyhda) of the Amphioxug. (Cf. Fig. 1.) 



Fig. 8. — An amphioxus-egg in the process of cleavage. (Cf. Fig. 2), 



Fig. 9. — Blastula of the Amphioxus. (Cf. Fig. 3.) 



Fig. 10.— Gastrula of the Amphioxus. (Cf. Fig. 4.) 



Fig. 11. — Young larva of the Am[)hioxus. The notochord {ch) lies 

 between the medullary tube (m) and the intestinal tube (d). The medullar^; 

 tube has an opening at the anterior extremity of the body (ma). 



