336 DOMESTICATED TROUT. 



bral divisions. The space corresponding to the mesen- 

 cephalon is the largest, and in profile it is easily discerned 

 by its enlargement, which begins near the ocular lobes. 

 The dorsal furrow is closed in tube form on the back. 

 The vertebral divisions begin to show, but almost exclu- 

 sively upon the front turned against the yolk. The part 

 of the dorsal furrow which is still open is very narrow. 

 The cells of the epidermoidal stratum have lost their nu- 

 cleoli and represent a tessellated epethelium. At the place 

 where the dorsal cord shows, cells are to be seen, filled 

 with an opaque and granulated alimentary substance. 

 From the thirteenth to the sixteenth day the three cere- 

 bral divisions are characterized in the most distinct man- 

 ner. The ocular lobes of the mesencephalon become more 

 and more distinct and completely enclosed on the side of 

 the mesencephalon as in a vault. The dorsal cord ap- 

 pears under the form of a simple string, solid and trans- 

 parent, in the middle of the embryo. The vertebral divis- 

 ions are perfectly distinct. The caudal extremity of the 

 embryo is circumscribed on the side of the yolk. The 

 yolk is surrounded on all sides by the epidermoidal stratum. 

 The yolk cavity has disappeared. Towards the end of 

 the sixteenth day the beginning of the crystalline coating 

 can be remarked in the eye. The rudiment of the ear ap- 

 pears at the same time under the form of an elliptical 

 vesicle, with a clearer space in the centre ; it is situated 

 a little in front of the nuchal bow. This, as well as the 

 cephalic bow and the curvature of the trunk, is well marked 

 out. The yolk vesicle, on the contrary, is very much re- 

 duced. The epencephalon shows a few swellings analo- 

 gous to the " ganglions persistans des Trigles." 



From the seventeenth to the twentieth day the tail be- 

 gins to show itself, and the embryo uses it to give vigor- 

 ous shakes by striking with it laterally. The cephalic bow 

 becomes level. The crystalline coating develops and har- 



