46 THE EMBRYOLOGY OF THE HONEY BEE 



and is much closer. The lateral folds continue to approach one 

 another until they meet, their first point of juncture being natur- 

 ally that of most rapid movement, a short distance caudad of 

 their anterior ends, which up to this time remain nearly station- 

 ary. The process of meeting and fusion now progresses both 

 forward and backward from the first point of juncture, the 

 anterior portions of the lateral plates being very quickly united, 

 while the posterior come together rather slowly. Figures V and 

 VI illustrate two stages in the completion of the process of clos- 

 ing; in the former figure a narrow cleft extending nearly one- 

 half of the length of the germ band separates the lateral plates ; 

 in the latter figure this cleft is insignificant. With the final clos- 

 ure of this cleft the formation of the germ layers may be con- 

 sidered complete. 



Examination of sections of the stages described shows that 

 the process just described consists essentially in the depression 

 of a median area of the ventral plate — the middle plate, — and its 

 overgrowth by the lateral portions of the ventral plate — the lat- 

 eral plates, — which have broken away from its edges along the 

 line indicated by the lateral folds at the time of their first ap- 

 pearance. In respect to the manner of formation of the meso- 

 derm the observations recorded above are in complete agreement 

 with those of Kowalevski (1871) and Grassi (1884). As al- 

 ready mentioned (p. 40) the beginning of the process of meso- 

 derm formation ("gastrulation," so-called) is indicated in figure 

 18B. In this figure a median section of the ventral blastoderm 

 is seen to be in process of separation from the lateral portions 

 by the mere displacement of the cells on each side of the bound- 

 ary line. That is, the cells forming the margin of the nascent 

 middle plate appear to be sliding inward over those forming the 

 edges of the future lateral places. This stage is the earliest ob- 

 served. Figure 19 represents a transverse section through the 

 anterior end of an egg at Stage IV. Here the middle plate has 

 become still more depressed, while the edge of the lateral plate 

 on the left side of the figure has already overlapped the cor- 

 rsponding edge of the lateral plate ; on the right side of the 

 figure the relation between the middle and lateral plates is much 

 the same as in figure 18B. In sections like that represented by 

 figure 19 it is evident that the middle plate has been actually de- 



