ox THE DEVELOPMENT OF LIMULUS T-OXGISPIXA. 63 



On August lltli (14 clays after fertilization, cf. fig. 5), the meso- 

 blast layer had divided into two lateral bands (fig. 25). Since the 

 preceding day the mesoblast had already divided into as many trans- 

 verse bands as there are segments externally seen. 



Late on August 12th (15 days after fertilization, cf. fig. 7 J, the 

 mesoblast of the fifth appendage-bearing segment had split into two 

 layers, somatopleur and splanchnopleur, producing a cavity — the 

 coelomic cavity — between them (fig. 26). As the mesoblast is divided 

 into two lateral hahes the cavity is of course paired. The fifth 

 appendage-bearing segment was well developed, and was sepai'ated 

 from t*lie fourth and sixth a])pendage-bearing segments by deep 

 grooves. 



On August 13th (16 days after fertilization, cf. figs, 8 & 9). a 

 short median groove had formed in the posterior part of the ventral 

 plate. Cells underlying the groove had become loose, wandered in, and 

 expanded into a sheet between the epi blast and the yolk-hypoblast 

 (fig. 27). I shall call the groove the primitii'c streak. The epiblast 

 cells of the ventral plate and most of the mesoblast cells have bv this 

 time lost their nucleolus, while the epiblast cells of the dorsum and 

 hypoblast cells liave still distinct nucleoli (fig. 41). The mesoblast 

 of the sixth appendage- bearing segment had split into two layers, 

 producing the cœlomic cavity. 



On August 14th (17 days after fertilization, cf. fig. 10), a pair of 

 the coelomic cavities, common to the cephalic lobe and the first 

 appendage- bearing segment, had appeared (figs. 29, 30), and another 

 pair in the seventh appendage-bearing segment (fig. 31). The pri- 

 mitive streak had receded partly towards the posterior end of the ventral 

 plate. The mesoblast produced from the primitive streak had separated 

 into two lateral halves and then divided into many transverse seg- 

 ments. I am inclined to believe that the mesoblast belonging to the 



