538 



cephalic plate, the primitive groove extends across the somites, its 

 anterior end terminating at the mouth while posteriorly it runs into 

 the caudal plate; the caudal plate is much smaller than in Balfour's 

 figure. 



5) Just after six somites are formed paired thickenings, the rudi- 

 ments of legs arise near the outer margins of each. The six pairs arise 

 simultaneously. I have seen no traces of O shorn 's semicircular 

 groove. 



6) Almost simultaneously with the outgrowth of the legs paired 

 thickenings for the nervous system appear. There are a pair of these 

 in each somite of the body while three pairs appear in the cephalic 

 plate. A few days later a series of six pairs of segmentally arranged 

 sensory thickenings arise outside of the legs, and extend in a line from 

 the cephalic lobes backwards, as briefly described by Patten^. These 

 have different fates. The first pair gives rise to the median ocelli of 

 the adult; the second to a peculiar sense organ as yet undescribed oc- 

 curring on the thin skin just in front of the first pair of appendages ; 

 the third soon disappears; the fourth forms the »dorsal organ« of 

 Watase, which persists longer than the third; the fifth gives rise to 

 the paired compound eyes; while the sixth is evanescent. At first 

 these are all similar and are plainly sensory. These organs are con- 

 nected with each other and with the brain by a longitudinal nerve 

 which takes an undulating course between the organs and the bases of 

 the legs. 



7) There is a precocious separation of ectoderm and entoderm 

 (yolk cells) during the formation of the blastoderm. Blastopore and 

 primitive groove produce no invagination of entoderm cells. The ento- 

 derm retains its primitive character as a solid mass of large yolk cells 

 until after the caudal spine appears. The yolk cells are not true vitello- 

 phags. They metabolize the yolk which is contained in each, but the 

 cells themselves are directly converted into the living epithelium of 

 the mid gut. By this process a lumen is formed, first at the anterior 

 end. The stomodeal-mesenteric wall is first to break through; the 

 opening into the proctodeum appears much later. The proctodeum is 

 very short, not extending far from the anus. 



8) In embryos at the time of hatching the sternal artery has ar- 

 rived at the condition found in the adult scorpion. It consists of a tube 

 lying on the upper surface of each half of the oesophageal nerve ring. 

 Not until much later than my studies have gone does it attain the in- 

 vesting character of the adult. 



^ Journ. Morphology III. 



