312 DOMESTICATED TROUT. 



The tail grows larger. The first traces of the odd fin are 

 formed by the epidermoidal layer upon the circumference 

 of the embryo ; the yolk vesicle disappears. 



From the twenty-third to the twenty-seventh day the 

 first rudiments of the nasal cavities are seen to appear on 

 the lower front of the head. The prolongation of the pro- 

 sencephalon, forming the olfactory nerve, stretches so as 

 to reach the nasal cavities. The head rises, disengages 

 itself from the yolk in consequence of the strengthening 

 of the nuchal bow. The yolk begins to separate from the 

 head, and the separation of the belly follows at the same 

 time that an uneven ventral fin shows, formed from the 

 epidermoidal layer. The choroid surrounds nearly all the 

 bulb of the eye ; the coloboma of the iris appears under 

 the form of a light cleft. The thick blastema of the base 

 of the cranium is very distinct. In the dorsal cord the 

 little cells develop in the form of little isolated vesicles, 

 which increase and occupy all the cord in front and be- 

 hind. The blood-producing layer extends over the yolk. 

 The choroid can be recognized by the naked eye in con- 

 sequence of the accumulation of pigment, and the eyes 

 can be distinguished through the shell membrane under 

 the form of two black points. The intestine and the 

 urethra are transformed into complete tubes, not showing 

 any trace of cellular structure. The anus is still closed. 



From the twenty-seventh to the thirtieth day the pineal 

 gland appears in the form of a little globulous accumula- 

 tion of cells in the semicircular cavity situated behind 

 the prosencephalon. The interior formations of the me- 

 sencephalon begin to show. The thick blastema of the 

 base of the cranium contracts very distinctly in the neigh- 

 borhood of the hypophysis The ear is much nearer to 

 the eye than formerly. The first traces of circulation ap- 

 pear in the beginning, under the form of two similar cur- 

 rents, one of which is destined for the head and the other 



