452 ]VJr. E. E. Prince on the 



is often compound, and presents tlie appearance of a group 

 of bubble-like structures enveloped bj a thin protoplasmic 

 stratum undistinguisliable from the protoplasm of the under- 

 lying periblast. In some Teleosteans it can be made out 

 very early ; according to Henneguy it appears in Salmo fayno 

 at the time when the blastopore coincides with the equator 

 of the vitellus ; but usually its appearance immediately pre- 

 cedes or succeeds the closure of the blastopore. 



Sense-organs, Heart, Coeloma, Wolffian Ducts, &c. 



In the evolution of the sense-organs few special features 

 can in this place be noted. The optic vesicles are always 

 rapidly budded off when the cephalic enlargement of the 

 neurochord is defined. They are solid and somewhat ovoid, 

 and their cells soon show a radial disposition, as though about 

 to dehisce along a central vertical plane in order to form a 

 median chamber, longitudinally placed. The formation of 

 this chamber — the cavity of the primitive vesicle — is never 

 accomplished, and only when the ingrowth of epiblast and the 

 formation of the concentrically laminated lens pushes the ex- 

 ternal portion inward upon the inner portion of the hulbus ocuU 

 is a cavity formed within the vesicle, the so-called secondary 

 vesicle. By this involution of superficial epiblast the rim of 

 the secondary cup is left imperfect upon its ventral margin, and 

 this breach is the choroidal fissure. The olfactory diverticula 

 are pushed out as modifications of the brain a little later, and 

 on reaching the epiblast in front of the head a ganglion is 

 formed, uniting with an epiblast thickening, from which the 

 olfactory nerve, according to Beard *, is split off, and thus, 

 like the cranial nerves, is partly epidermal. 



Like the eye, the ear in the Teleostei originates as a solid 

 diflerentiation of cells. The otocysts can be distinguished 

 twenty or thirty hours after the blastopore has closed. They 

 are ovate in form, and rapidly develop a lumen, which is at 

 first a narrow fissure surrounded by dense epiblast. The 

 lumen rapidly enlarges, the walls become thinner, and before 

 the embryo emerges from the ovum each otocyst develops two 

 calcareous refringent otoliths, which exhibit a marked radiate 

 structure. 



The heart is a prominent structure in the early embryo 

 and protrudes as a solid mass of splanchnic-mesoblast cells in 

 the centre of the pectoral region, antero-ventrally situated 

 below the otocysts. For some time it is solid and function- 



* Beard, " Branchial Sense-Organs of the Ichthyopsidii," Quart Journ. 

 Micr. Sci., Dec. 1885. 



