THE PRIMITIVE SENSE BUDS. 



51 



and rod-like ends as those in the segmental sense organs on the outer margins of 

 the coxae. (Fig. 74, E.) 



The primitive sense buds appear as soon as the six thoracic appendages are 

 outlined (stage B), and are at first uniformly distributed over the entire cord and 

 cephalic lobes, with the sole exception of the olfactory lobes. (Fig. 15.) At 

 a later stage, E, those on the lateral margin of the cord are distinctly larger than 

 the rest, forming two dark bands. From the buds on the posterior lateral margin 

 of each neuromere, arise the ganglion cells at the roots of the post-thoracic nerves 

 (spinal ganglia). The buds on the smaller, or originally posterior segment of 

 the neuromere give rise to the cluster of motor nerve cells which are found near 

 the anterior nerve roots. 



aKA P^'^' 



pKn 



paen 



'/ " I r I' 



Fig. 42, — Brain of adult scorpion, from the side. 



As development proceeds, the central cavity of the bud closes, the sensory 

 cells lose their cylindrical form, and their hair-like, or rod-like outer ends disap- 

 pear; finally each bud forms a small cluster of ganglion cells. 



In the late embryonic stages of the scorpion, the metamorphosed sense buds 

 form long conical masses of cells with the proliferating apices directed inward. 

 Their appearance is then much like the cell clusters formed by neuroblasts. 

 (Figs. 227-228.) 



Cell division in the sense buds diminishes after their metamorphosis, the 

 ganglion cells reaching an approximately fixed number at an early embryonic 

 period. This, however, does not apply to the minute cells in the hemispheres, 

 in the olfactory lobes, or in the pedal ganglia of Limulus, for these cells appear 

 to increase in number steadily, at least as long as the animal continues to grow 

 in size. 



In Limulus the cells descended from a given sense bud, during the late larval 

 periods, form well defined clusters of pear-shaped ganglion cells, with a special 

 neuroglia investment. Each cell of the same cluster appears to project its fibers 

 along the same path, to the same terminals. (Figs. 61-64.) 



In Limulus it has not been possible to identify each nerve-cell cluster with 



