ARTHROPODA. 



105 



perceives the form of external objects, it must be by the combined 

 action of the ommatidia, which almost certainly do not act like 

 so many simple eyes. 



DEVELOPMENT. 



1. Early Stages (Fig. 31). The ovum is centrolecithal, con- 

 taining a central mass of food-yolk. Cleavage (segmentation) is 

 regular, but incomplete and peripheral, chiefly concerning the 

 external protoplasmic part of the oosperm. This is owing to 

 the food-yolk; but this, though it impedes division, is for the 

 most part affected by the cleavage, being divided into radiating 



Fig. 31. BLASTULA AND GASTRULA OF CRAYFISH (after Reicheribach and 

 Huxley). A, Blastula (segmented ovum); bl, blastoderm; v, yolk. 

 B, Gastrula ; ep.b, ectoderm (epiblast) ; m.g, archenteron (which 

 becomes the mid-gut), bounded by endoderm (hypoblast), shaded 

 darkly ; bp, blastopore. 



yolk-pyramids, continuous at first (A) with the segments of the 

 peripheral protoplasm, but -soon (B) becoming separated. The 

 blastula, at the end of cleavage, consists of a central mass of 

 yolk (v), enveloped by the blastoderm, a single layer of small 

 cells (W), which become thickened over a small oval area, the 

 germinal disc, that marks the ventral surface of the embryo. The 

 posterior part of this is invaginated (B) to form a small pouch 

 (m.g) sunk in the food-yolk. The embryo has now reached the 

 gastrula stage, and is practically a double-walled pouch, the inner 

 wall of which is the endoderm. (hypoblast) and the outer wall the 

 ectoderm (epiblast), while its small cavity is the archenteron, and 



