l62 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



Although the minute structure and the function of this organ need further 

 study, there is no question that it is a true sense organ of great morphological 

 significance. 



The Development of the Olfactory Organ and Nerves. — The Olfactory 

 Placodes. — The primary olfactory organ of Limulus represents a segmental sense 

 organ serially homologous with the lateral eyes and the ocelli. It is first seen as a 

 pair of sensory thickenings on the anterior margin of the lateral eye ganglion, 

 behind the median eye tubes. (Fig. 141, ol.o.) It is connected with the middle 

 lobe of this ganglion by the lateral olfactory nerve. (Figs. 36-39.) 



Each organ soon separates bodily from the ectoderm. Although there is no 

 visible infolding, the cells which have the appearance of visual cells, are in\erted 

 in the process and become filled with a dense mass of white pigment (guanin ?) . 

 (Fig. 37, A.) At the same time certain cells filled with the same kind of pigment 

 migrate forward from each placode, forming a gradually widening, sub-epithelial 

 plexus of branching pigment cells connected with the anterior margin of the 

 placode by a short thick stalk. (Fig. 36, p.st.) During the early embryonic 

 stages the placodes move toward the remnants of the anterior neuropore and 

 there unite in the median line, meantime acquiring a connection with the anterior 

 surface of the cerebral hemispheres and the olfactory lobes. (Fig. 142.) 



Lateral Olfactory Nerve. — In the following stages, the united placodes move 

 forward beneath the integument toward their position in the adult. During 

 this process, the lateral olfactory nerves become greatly elongated and the cells 

 of the original placodes are now scattered as ganglion cells along the nerve, 

 but forming a special enlargement at either end. These terminal masses consist 

 of irregular clusters of five or six large pear-shaped cells which greatly resemble 

 the ommatidial cells of the paired ocelli, not only in their shape and arrange- 

 ment, but in the presence of the clear refractive rods, or rhabdoms, on their side 

 walls. (Fig. 109, C.) 



In young Limuli (2-3 in.), the peripheral end of the lateral olfactories still 

 terminates in a compact, club-shaped mass of metamorphosed visual cells. (109, 

 B.l.ol.n.) It also sends out several fine nerve branches which ramify widely under 

 the skin, in the region surrounding the main olfactory organ. (Fig. 70.) At 

 the same time the terminal group of cells breaks up into irregular clusters scattered 

 among the branches of the nerve. In the ordinary methods of preparation, each 

 cluster has the appearance of an isolated ommatidium composed of large pear- 

 shaped ganglion cells, whose proximal ends form coarse nerve tubes. There is 

 another group of cells, similar to those just described, scattered along the proxi- 

 mal end of the main nerve, some of them outside the brain sheath, but the major- 

 ity within it, on the root of the nerve as it passes over the surface of the hemi- 

 spheres. (Figs. 39, 48, 51, 66, 109, ol.l.n. and gc^ and go''.) 



Both these cell groups, which contain granules that have a glistening white 

 appearance in reflected light, are the modified descendants of the cells constitut- 

 ing the original visual placode. Even in the adult, they still show traces of 



