;8 THE EMBRYOLOGY OF THE HONEY BEE 



in origin, since their manner of formation, although essentially 

 similar to that of the mesoderm, differs much from it in detail, 

 moreover the cells composing the mesenteron rudiments are from 

 the first distinguishable from those of the mesoderm. A final 

 decision between these two interpretations seems premature. The 

 honey bee is a highly specialized member of a specialized order, 

 and therefore an unsuitable form on which to base generaliza- 

 tions, since its development certainly presents many modifica- 

 tions of the type, moreover generalizations based on the study 

 of one form are always unsafe. 



It is sufficiently evident that in spite of the numerous papers 

 dealing with the origin of the mesenteron in insect embryos, 

 there is much need of further investigation, particularly of 

 the more generalized types. Superficial study, however, would 

 be worse than useless ; the type of investigation demanded is the 

 highest, requiring the delicate and precise methods of the cytol- 

 ogist, the best fixation and staining possible, a complete series 

 of stages, a study of the origin of the rudiments cell by cell, and 

 finally an eye single to the facts and regardless of preconceived 

 theoretical considerations. 



Before leaving the subject of the germ layers it will be neces- 

 sary, in order to discuss the stages following, to describe briefly 

 the structure of an embryo at the final stage of this period, Stage 

 VII. This stage is illustrated by a series of transverse sections, 

 represented by figures 29, 30, 31 and 32. 



Figure 29 shows a section passing through the extreme an- 

 terior end of the embryo. The ectoderm here extends over about 

 two-thirds of the circumference of the yolk and is much thick- 

 ened in its lateral portions. Within the ectoderm is a crescentic 

 mass of cells, the anterior mesenteron rudiment, which has 

 grown both cephalad and laterad to form a cap-like mass cover- 

 ing the ventral side of the cephalic end of the yolk. In this and 

 also in the next section the amnion (Am) is seen covering the 

 exterior of the embryo as a thin sheet of flattened cells. This 

 membrane will be discussed at length later. 



Figure 30 also shows a section passing through the cephalic 

 end of the embryo, intersecting it just caudad of the posterior 

 limits of the anterior mesenteron rudiment. The opening in the 

 ectoderm through which the anterior mesenteron rudiment reaches 



