218 THE EMBRYOLOGY OF THE HONEY BEE 



the fifth abdominal. Two points of difference are thus to be noted 

 between the mason bee and the honey bee. In the former the 

 rudiments of the sex glands are formed from the visceral wall 

 of the mesoderm mesiad (ventrad) of the visceral wall of the 

 mesodermal tubes, instead of from the visceral wall of the tubes 

 themselves. Moreover the rudiment originally extends through 

 but three of the abdominal segments instead of five, and contracts 

 so as to lie in but one segment, instead of three. 



Up to the year 1895 the belief prevailed among embryologists 

 that the genital cells in insects were as a rule derived directly from 

 mesoderm cells. To this rule however several important excep- 

 tions had already been noted ; in all these cases the genital cells 

 were derived not from the mesoderm, but from special cells 

 set aside at an early period in the development. As early as 1865 

 Leuckart and Metschinkoff discovered that the genital organs in 

 Cecidomya were formed from a group of cells (the "pole cells" of 

 Robin and of Weismann) derived from a single cleavage cell 

 originally situated at the posterior pole of the egg and readily 

 distinguishable from the other cells during the early stages of 

 cleavage. This discovery was confirmed by Balbiani (1882, 1885) 

 and Ritter (1890) in Chiromomus. Metschnikoff as early as 1866 

 found that the cells destined to form the genital organs were dis- 

 tinguishable at an early period of development in Aphis, and 

 this observation has been confirmed by later investigators (Wit- 

 laczil 1884, Will i888a), while Woodworth (1889) and Schwan- 

 gart (1904) obtain similar results in the case of the butterflies. 

 In 1895 Heymons described the sex cells of Forficula and several 

 members of the Orthoptera as being readily distinguishable at 

 about the time of the formation of the mesoderm, and located 

 at the posterior end of the germ band. They subsequently migrate 

 cephalad along the somatic layer of the mesoderm to their defini- 

 tive position. Soon after the publication of Heymons' researches, 

 Lecaillon (i897a) announced that in certain chrysomelid beetles 

 the sex cells could be distinguished during the formation of the 

 blastoderm as a group situated at the posterior pole of the egg 

 and differing from the ordinary blastoderm cells in their larger 

 size and darker stain. Their subsequent migration is similar to 

 that of Forficula and the Orthoptera. These results have been 



