70 THE EMBRYOLOGY OF THE HONEY BEE 



mesenteron at later stages is seen lying close beneath the lateral 

 portions of the mesoderm. 



Grassi (1884) gave an essentially correct account of the origin 

 of the mesenteron in the honey bee, and one which has been 

 generally accepted as such. His observations of the development 

 of the anterior mesenteron rudiment were much more satisfac- 

 tory and complete than those of the posterior rudiment. The 

 substance of his statement in regard to it is as follows : After 

 the formation of the mesoderm (middle-plate) has begun, the 

 median part of that area of the blastoderm anterior to the fur- 

 rows (lateral folds) becomes many layered, with the exception 

 of its anterior margin. Later, beginning at the lateral margin 

 of the many-layered portion, and perhaps also at its anterior 

 margin, the superficial layer is separated from the deeper layers. 

 This superficial layer is continuous with the one-layered blasto- 

 derm at the anterior pole, and is also continuous caudad with 

 the ectoderm and is itself ectoderm. The deeper layers are con- 

 tinuous caudad with the mesoderm and are themselves meso- 

 derm. The formation of the posterior mesenteron rudiment was 

 supposed to be similar to that of the anterior rudiment. One 

 entire plate (PL VI) containing thirty-six figures of sections is 

 devoted to the development of the mesenteron rudiments. It 

 is to be noted, however, that Grassi makes no statement as to 

 the manner in which these rudiments are produced, and also 

 failed to note the break in the ectoderm covering the anterior 

 mesenteron rudiment, where the mouth is formed later. More- 

 over, his belief that these rudiments are to be interpreted as 

 mesoderm is, in view of subsequent investigation, scarcely justi- 

 fied; nevertheless his account of the origin of the mesenteron 

 rudiment is probably the most important part of Grassi's paper, 

 and also the one which attracted most attention. 



Carriere and Burger's (1897) account of the formation of the 

 mesenteron rudiments of the mason bee is substantially the same 

 as that of Grassi, differing from it principally in interpretation. 

 According to these investigators the anterior and posterior me- 

 senteron rudiments arise from proliferating areas of the un- 

 differentiated blastoderm at the two ends of the ventral plate, 

 respectively cephalad and caudad to the middle plate and in- 

 dependent of it. These rudiments constitute two large hemi- 



