30 WILLIAM PATTEN. 



c. The Development of the Olfactory Organs. — The 

 olfactory organs first appear in surface views as a pair of oval 

 ectodermic thickenings, the ^'primary olfactory organs'' on the 

 lateral margin of the brain, just in front of the optic ganglion 

 (figs. 24, 25, 46, 47, 49, and 61). Each organ soon separates 

 from the brain and grows forward, leaving behind long, thick 

 strands of ganglion-cells, which constitute the lateral olfactory 

 nerves (fig. 25). As soon as the primary olfactory thickenings 

 are separated from the brain they become filled with white 

 pigment ; at the same time branched cells filled with white 

 pigment leave the thickening, and, extending under the ecto- 

 derm in all directions, form a gradually widening pigmented 

 plexus; that in the adult maybe several square inches in 

 extent. In the young larvae the pigmented or choroid 

 plexus is attached to the anterior edge of the primary olfac- 

 tory thickening by a stout stalk (figs. 46 and 47, p. st.). Each 

 plexus lies beneath the ectoderm and a little in front of the 

 brain, which even at this stage it more than equals in size. At 

 a little later stage the two plexi become completely united 

 (fig. 45). In fig. 22, w. p., a part of the plexus is shown in 

 section under a higher power. 



The primitive olfactory thickenings soon unite in the median 

 line to form an apparently unpaired organ lying some dis- 

 tance in front of the brain. Before this takes place the cells 

 constituting the thickening arrange themselves in irregular 

 clusters, and their walls develop those peculiar cuticula-like 

 thickenings that look so much like groups of visual rods in the 

 ommatidia. Some of the^ cell-clusters soon leave the main 

 thickening, and lie scattered about under the ectoderm, but 

 connected with the distal portion of the lateral olfactory nerve 

 by branching nerve-bundles (fig. 18, x); others remain in the 

 distal end of the nerve to form the terminal ganglionic swell- 

 ing. In specimens about two inches long there is nothing 

 left of the primitive olfactory thickening but an irregular mass 

 of large cells, mostly pear-shaped, in which the lateral olfactory 

 nerves terminate. There is nothing in the final position, 

 shape, or structure of these cells to indicate that they now 



