THE EMBRYOLOGY OF THE HONEY BEE 249 



"In the first period are included : the formation of the embryo, 

 of the rudiments of the cephalic lobes and of the embryonic 

 envelopes up to their complete overgrowth of the embryo. 

 At the end of this period the completely segmented germ 

 band has extended its entire length over the surface of the 

 yolk. To the second period belongs : the invaginations con- 

 stituting the proctodaeum and stomodaeum, the formation of the 

 rudiments of the appendages and of the central nervous system, 

 and the tracheal invaginations. Into this period falls the sep- 

 aration of the entoderm from the mesoderm together with the first 

 rudiments of the mid-intestine, the appearance of the primary seg- 

 ments, the development of the definitive body cavity and the 

 segmentation of the yolk. The rupture of the embryonic mem- 

 branes and their withdrawal to the dorsal side of the egg, and an 

 invagination, introducing the formation of the dorsal organ, form 

 the limits between the second and third embryonic periods. . . . 

 No new organs are produced, this period serves principally for 

 histological differentiation." 



In dividing the development of the bee into three corresponding 

 periods, indicated in the table given above by double lines, it is 

 easy to determine the limit between the first and second periods, 

 which falls between Stage VII, forty-two to forty-four hours, and 

 VIII, forty-four to forty-six hours. The limit between the sec- 

 ond and third periods is not so easy to find, since there is in the 

 bee but a single embryonic membrane, which is not ruptuied until 

 hatching and which forms no "dorsal organ." Moreover in the bee 

 egg there is no movement of the embryo corresponding to "revolu- 

 tion" or "blastokinesis" (Wheeler, 1893). By referring to Hei- 

 der's table however, it is seen that at the end of the second period, 

 which falls between the fourth and fifth days of development, the 

 tracheal invaginations, formed but a few hours previously, are still 

 cleftlike. During the fourth day moreover the invaginations of 

 the proctodaeum and stomodaeum are formed. In the bee there- 

 fore the limit between the second and third periods has been fixed 

 between Stages IX and X, corresponding respectively to the ages 

 of forty-eight to fifty hours and fifty-two to fifty-four hours, 

 which is probably sufficiently close for purposes of comparison. 



In Hydrophilas the time required for complete development is 



