THE EMBRYOLOGY OP A SCOEPION. 115 



cells in it (fig. 12, y. c.) are numerous. Their nuclei are very 

 large and granular, and of irregular shapes. The cell-outlines 

 have entirely vanished, the cells being swollen up by an enor- 

 mous quantity of yolk-stuff. According to Kowalevsky and 

 Schulgin these cells are capable of amoeboid movements. Cells 

 continue to be added from the under surface of the blastoderm 

 to those already in the yolk up to the end of this stage. Their 

 function — of breaking down the yolk — is carried on at a later 

 period by the hypoblast. 



Stage E. — In the next stage the blastoderm (PI. XIV, 

 fig. 13) has assumed an oval form, the thickened part or 

 ventral plate measuring '35 mm. in length and '25 mm. in 

 breadth, though the peripheral cells extend some way beyond 

 this. I have not been able, either in surface view or section, to 

 find any trace of the primitive groove, and imagine that, if ever 

 present, it has filled up. The primitive thickening (fig. 14, pr. t.) 

 is better developed than in the last stage, and the single layer 

 of primitive hypoblast (figs. 14 and 15, pr. hy.) is now quite 

 definite and extends a little way beyond the thick part of the 

 blastoderm, and forms a layer {hy'.) of cells under the peri- 

 pheral cells. These last {s. m' .) extend a good deal further than 

 in the last stage. The serous membrane {s. m.) is now com- 

 pleted over the surface of the ventral plate. 



Stage F. — In the next stage the embryo, of which fig. 16 

 shows a longitudinal section, consists of two somites — those 

 which will afterwards bear the chelicerse and chelae — in addition 

 to the head- and tail-segments. The head- and tail-segments 

 are large, and a third somite is beginning to be formed from 

 the tail. The first somite is smaller than the second, and not 

 as yet very distinctly marked off from the head. It does not 

 become fully separated from the head until a much later stage 

 (eight somites). Except for this curious delay in the forma- 

 tion of the first, all the somites are formed and separated in 

 regular succession from the tail-segment. 



The epiblast has undergone little change since the last stage, 

 except that it is somewhat thinner between the somites than in 

 them. It is beginning to grow up at the edges over the surface 



