THE EMBRYOLOGY OP A SCORPION. 117 



that of Kowalevsky and Scliulgin. They describe it as a fold, the 

 outer layer of which forms the serous membrane while the 

 inner forms the amnion. This is probably the more primitive 

 mode of origin for these structures, and the mode described 

 above for E. italicusis probably derived from it either by a 

 hastening of the formation of the serous membrane or a retarda- 

 tion of that of the amnion. I am unable to confirm their state- 

 ment that mesoderm cells are present between the two layers. 



Third Period. — Up to the Formation of Nine Somites. 



This period covers the rest of the time before the appendages 

 begin to form. The egg has by this time entirely passed into 

 the ovarian tube. It has also increased considerably in size, 

 but I am unable to say whether this is due in any degree to 

 absorption of fluid or whether it is entirely due to internal 

 changes. 



Stage G. — In the first stage belonging to this period which 

 I have examined (PI. XV, fig. 20) the embryo consists of 

 nine somites. The first of these — that which will bear the 

 chelicerae, is much smaller than the others, and is seen in section 

 to be not yet fully separated from the head. The second 

 somite, which will bear the chelse, is larger than those following 

 it. The next four are the ambulatory, and the seventh will 

 bear the genital operculum. A slight groove (w. (j.) runs 

 down the middle line of the body ; this is chiefly due to 

 the mesoblast having divided into two longitudinal bands 

 (figs. 21 and 22, mes.). 



The epiblast is moderately thick in the somites, and is 

 beginning to grow as a single layer round the rest of the egg 

 (fig. 21, ep' .), carrying the amnion with it. By this stage it 

 has extended almost as far as has the hypoblast. The cells in the 

 middle line show a more definite arrangement than the rest of 

 the epiblast. This is preparatory to the formation of the 

 neural groove. The cells of the amnion [am.) have developed 

 their characteristic nuclei — spindle-shaped in section — and 

 form a well-marked thin membrane lying close over the embryo. 



The mesoblast (figs. 21 and 22, mes.) shows most impor- 



