54 



THE EMBRYOLOGY OF THE HONEY BEE 



regions display a sort of secondary segmentation induced by 

 that of the ectoderm. The ^esoderm is here of nearly uniform 

 thickness, in the later stage (VI) double-layered (Fig. 236), and 

 thrown into a series of low transverse folds by the segmental 

 swellings of the overlying ectoderm. Near the mid-ventral line 

 the segmental swellings of the mesoderm are intact although in- 

 conspicuous, but instead of corresponding with those of the ecto- 

 derm they fit into the intersegmental depressions. 



In general it may be said that up to Stage VIII the segments 

 are not uniformly well defined, and their boundaries not sharp, 

 especially at the two ends of the germ band, so that certain 

 identification of individual segments is difficult if not impossible. 

 Those segments chosen for illustration were unusually well de- 

 fined. All that can be safely affirmed is the presence of seg- 

 mentation at this stage. 



Biitschli (1870) observed and correctly interpreted these early 

 evidences of segmentation, although neither Kowalevski nor 

 Grassi appear to have noticed them. Biitschli's account is of 

 sufficient interest to quote, and is as follows : "In addition to 

 these primary rudiments of the germinal ridges there are also 

 found the first indication of the segments. I had long overlooked 

 this precocious process, until I investigated the finer structure of 

 the germ band with a high magnification. In contrast to earlier 

 stages this shows no longer the regular cellular structure, but 

 transverse bands, in which the cells are pressed closely together, 

 alternating with the others which are elongated and with their 

 long axis placed in a transverse plane. On closer examination 

 one notices that this condition on the surface harmonizes with 

 the image seen in optical section, which shows swellings alter- 

 nately with thinner portions, where it is one layered, as well as 

 where it is many layered. Frequently in the contracted portion 

 between two swellings there appears a dark transverse line, 

 which seems to indicate a cleft through the entire thickness of 

 the germ band. The bands described, composed of closely packed 

 cells, form the median portions of the segments, the somewhat 

 larger, more elongated cells lie in the boundary between two 

 neighboring segments. By raising and lowering the microscope 

 tube, I have often persuaded myself that the first bands lie at 

 a higher level than the part lying between them so that ac- 



