342 WHEELER. [Vol. IV. 



the single band of cells proceeding from each neuroblast ; the 

 daughter cells having divided, so that each teloblast surmounts 

 three or four rows of small cells. Graber, in a recent paper,i 

 calls attention to the huge cells that give rise to the ganglionic 

 thickenings in the Dipter Lucilia and in the Coleoptera Lina 

 and Melolontha. 



Of great interest in this connection are the figures given by 

 Dr. Patten in his last paper.^ The nervous system of the young 

 scorpion embryo is represented as covered with peculiar "sense- 

 organs," which, strange to say, are arranged in four irregular 

 rows in either lateral cord of the ventral nerve-chain. A sin- 

 gle large " sense-organ " occurs in the middle of the median 

 cord portion of each segment, and hence nearly corresponds in 

 position to the median neuroblast of XipJddiimt after it has 

 moved forward between the connectives. The " sense "-spots of 

 the scorpion are very similar to the spots which in the proce- 

 phalic lobes of the Locustid mark the small areas where neuro- 

 blasts are differentiating from the primitive ectoderm. Patten 

 gives no description of these "sense-organs" in his text, and 

 the cut representing a transverse section through a ventral 

 ganglion is on too small a scale to show the surface cells dis- 

 tinctly. Laurie ^ figures a transverse section through the pro- 

 cephalic lobes of an embryo in a stage that I take to corre- 

 spond to one of the young embryos figured by Patten. In this 

 figure (PI. XV, Fig. 25) the ectoderm shows in the arrange- 

 ment of its nuclei a peculiar unevenness which may account 

 for the circular spots figured by Patten, but which would seem 

 to have no connection with the differentiation of neuroblasts 

 and ganglion cells. Nevertheless, the striking agreement in the 

 number and arrangement of the neuroblasts of XipJddimn and 

 the " sense-organs " of the scorpion, a form in most respects so 

 far removed from the Insecta, warrants the suggestion that the 

 so-called sense-organs in the Arachnid are, like the circular 

 spots in the procephalic lobes of XipJiidhnn, simply the expres- 

 sion of an early differentiation of the primitive ectoderm into 



1 Vergleichende Studien, etc. Denkschrift. d. k. Akad. d. Wiss. Wien, Bd. LVI, 

 1889, p. 48. 



2 Quart. Journ. Micr. Sci., XXXI, 1890. 



3 Quart. Journ. Micr. Sci., Vol. XXXI, 1890. 



