ARACHN1DA. 



635 



dimeuts of the limbs of the embryo. The first change is the 

 formation of the "primitive streak," or the splitting of the 

 blastoderm, which is due to a local multiplication of the cel- 

 lules along the median line of the egg. 



These tubercles result from a simple thickening of the blas- 

 toderm, and what is ultimately destined to be the back (tergum) 

 of the animal, arises from a similar thickening of the blasto- 

 derm, which he calls the "primitive cumulus." This mass, 

 easily distinguished by its whiteness, always floats on the top 

 of the yolk of the egg, keeping its position next the eye of 

 the observer. The "cumulus," at first almost hemispherical, 

 elongates over the surface of the blastoderm, becoming pyri- 

 form. This region is the posterior, or anal, pole of the egg. 



Fig. 624. 



Fig. 623. 



Fig. 625. 



We see the "cumulus" spreading from the anal pole over the 

 surface like a veil, but it is less white than the polar region. 

 This veil continues to spread over the entire surface to a pole 

 opposing the anal, which Claparede terms the cephalic pole. 

 Each pole forms a very prominent projection. At this stage 

 the body of the embryo becomes well marked and subdivided, 

 worm-like, into rings. (Fig. 623.) The extent of the dorsal 

 region is greatly limited, while that of the ventral side is 

 greatly increased. 



The entire ventral region, occup^ying most of the whole egg, 

 is homologous with the primitive ventral streak. It is at this 

 time that the formation of the protozoonites (elemental rings, 



