THE EMBRYOLOGY OF THE HONEY BEE 261 



the formation of the protodaeum, grouped around the point 

 where the proctodaeum is to appear. Four separate invaginations 

 have not been observed, the pair situated on each side of the mid- 

 line being connected by a shallow crescentic groove. 



The primary yolk cells, which are derived from cleavage cells 

 remaining within the yolk, multiply by mitosis, the mitotic figures 

 being at first similar to those of the cleavage cells. A little later 

 irregular mitotic figures are found. These are usually minute, 

 and have the appearance of being in some cases multipolar, 

 and in others unequal. Multinucleate cells soon become abundant, 

 and some of these cells are of large size. Degeneration of the 

 nuclei of the yolk cells soon becomes frequent, such nuclei dimin- 

 ishing in size and finally becoming reduced to minute deep-staining 

 spherules which leave the cell body and enter the yolk. Sec- 

 ondary yolk cells are formed by the immigration of nuclei from 

 the blastoderm into the yolk. These soon become indistinguish- 

 able from the primary yolk cells. Yolk cells are found distributed 

 through the yolk until shortly before hatching. They are fre- 

 quently seen clustered under the epithelium of the mid-intestine, 

 during the time when the latter is engaged in covering the lateral 

 faces of the yolk. Yolk cells and yolk disintegrate at the time of 

 hatching, being presumably digested. 



The total time normally required for the development of the 

 egg is seventy-six hours. This is divided approximately as fol- 

 lows: Cleavage, fourteen to sixteen hours; formation of the 

 blastoderm, fourteen to sixteen hours ; formation of mesoderm, 

 rudiments of mesenteron and embryonic envelope, twelve to four- 

 teen hours ; remainder of development, including differentiation of 

 tissues and organs, thirty-two or thirty-four hours. The earlier 

 stages, including the formation of the so-called "germ layers," the 

 amnion, etc., occupy considerably over one-half of the time re- 

 quired for total development. 



