122 MALCOLM LAURIE. 



The ventral nervous system consists of a pair of tliickcued 

 bands of epiblast running the whole length of the body on each 

 side of the neural groove (PI. XV^ fig. 28). The bands are 

 cut up into blocks by the grooves which separate the somites. 

 The epiblast is not evenly thickened, but the nuclei are ar- 

 ranged so as to present a wavy outline. This is characteristic 

 of the formation of nerve-tissue in this animal, and was well 

 seen in the cerebral lobes in the last stage (PI. XV, fig. 25). 

 The small ganglia of the cheliceral somite are well seen at 

 this stage (fig. 28, g. I). 



The tail-segment, from which the six caudal somites have yet 

 to be formed, has begun to be pushed out (PL XVI, fig. 29). 

 The epiblast in this region is very thick, and the cavity of the 

 outpushing is lined by a thick layer of hypoblast, which is the 

 " hypoblastic mass '' of earlier stages (fig. 29, hy. m.). 



Besides this mass in the tail-segment the hypoblast extends 

 as a single layer round the whole egg (PI. XVI, fig. 29, hy.). 

 Along the ventral side the cells of this layer are close together, 

 but towards the sides and back they become more scattered, 

 and are to a great extent involved in the yolk. It is from the 

 mass in the tail-segment that the mesenteron is chiefly formed. 

 The hypoblast along the ventral surface also takes some part in 

 its formation, but that round the sides and back is not involved, 

 though it aids in the formation of the great digestive gland or 

 liver. 



The mesoblastic bands (PI. XVI, fig. 31) are not yet united 

 across the middle line. The coelomic spaces (PI. XVI, figs. 30 

 and 31) are well marked and quite separate for each segment. 

 Those in the first six somites are prolonged into the appendages. 

 The somatopleure is several cells thick ; the splanchnopleure, on 

 the contrary, consists of a single layer of cells. The mesoblast 

 in the cephalic segment is thinner than in the body somites, 

 and the coelomic space is narrower. 



Stage K (PI. XVI, fig. 32). — The thoracic appendages have 

 increased very much in size, and the chelicerfc and chelaj are 

 both bifurcated at the extremity. A section through the base 

 of one of the ambulatory appendages (PI. XVI, fig. 33) shows 



