1 64 THE EMBRYOLOGY OF THE HONEY BEE 



some length the probable homology of the "corpora incerta" and 

 arrives at the conclusion that these bodies probably represent the 

 tracheal rudiments of one of the cephalic segments and that they 

 furnish material for the tracheae of the head. No evidence to 

 support this view has been seen in the honey bee. On the con- 

 trary the corpora allata and the tracheae develop independently 

 of one another. In the honey bee the tracheae supplying the head 

 are derived from tracheal invaginations located on the second 

 maxillary segment (see p. 172). 



A possible clue to the homology of the corpora allata appears in 

 an interesting observation, ■ apparently hitherto unnoticed, by 

 Toyama (1900) on the embryo of the silkworm (Bombyx). In 

 the section devoted to the endoskeleton of the head Toyama states : 

 "In the mandibular segment three pairs of invaginations take 

 place; the most anterior (between the labrum and the mandible) 

 becomes the first tentorium, the second pair gives rise to the seat 

 of the extensor mandibulae, while the last becomes the flexor 

 mandibulae and salivary gland." 17 The flexor mandibulae and 

 salivary gland therefore arise from a common invagination, which 

 Toyama's figure (woodcut Fig. 1) shows to be situated at the 

 posterior margin of the base of the mandible, corresponding quite 

 closely to the point of origin of the flexor mandibulae and corpora 

 allata in the honey bee. This suggests that the corpora allata may 

 represent glands, vestigial in the honey bee and many other insects, 

 but functional in at least some of the Lepidoptera. Further 

 research in this direction is much needed. 



F. Degenerating Cells 



A phenomenon apparently peculiar to the bee, since it is not 

 mentioned by the investigators of other insects, is the frequent 

 occurrence of degenerating cells within the embryonic nerve tis- 

 sue. These cells occur isolated and in small number within the 

 ventral cord but in the brain they are abundant and to a certain 

 extent localized in definite regions. The largest, most conspicu- 

 ous and most constant of these include a pair of wedge-shaped 

 sections of the brain, one on each side, situated near the juncture 

 between the proto- and deutocerebrum, and including a part of 



17 L. c, p. 97. 



