238 AYERS ON THE DEVELOPMENT 



mesodermic elements. Furthermore, in this stage all the cells of the germinal hand are 

 provided with a single nucleus, and no trace of cell proliferation is seen in any part. As 

 favoring the existence of an invagination it may be observed, that some of the sections 

 show an arrangement of the cells which indicates a disturbance in their primitive position. 

 In pi. 22, fig. 28, is shown this disturbed arrangement. The outer ends of the cells on 

 either side the median line incline toward the latter. This arrangement of the cells is 

 similar to that found in sections of eggs of other insects just before and after the invagina- 

 tion has taken place. Moreover, the cells of this median region appear to be less regularly 

 columnar than those on either side. By far the greater portion of the mesodermic elements, 

 however, originate from yolk cells which migrate to the region of the germinal band 

 and there undergo division. 



The mesoderm has formed a continuous sheet over the inner side of the germinal band 

 before any modification in the form of the embryo appears. The almond-shaped thick- 

 ening is soon divided into two tolerably well marked regions by the enlargement of 

 the head end. The narrower portion of the germinal band increases somewhat in length 

 and the abdominal end becomes more broadly rounded, so that the embryo presents the 

 appearance shown in pi. 18, fig. 3. The mass of yolk substance during this time has 

 undergone important changes, due to the greatly increased number of yolk cells, which are 

 to be seen at this stage as grayish masses, larger than the yellow yolk globules and about 

 the size of the albuminoid masses. These occur at irregular intervals and by their trans- 

 parency help to clear up the yolk. The oil globules have decreased in number and size, 

 while there has been a proportional increase in the number and size of the albuminoid 

 bodies. 



With the further growth of the embryo the head lobes increase rapidly in breadth, the 

 ectoderm at the lateral edges becomes thicker, and the posterior portion of the embryo 

 becomes spatulate in form with the enlarged part of the spatula forming the tip. There 

 appears a depression in the middle* of the forehead which helps to make more conspicuous 

 the bilateral symmetry of the head region. It indicates the position of the future labrum 

 and forms the inner boundaries of the two cephalic ganglia, which are developed on either 

 side of this depression at a much later stage. Almost simultaneously with the appearance 

 of this depression, two lateral folds are formed in the spatulate portion of the embryo (pi. 

 18, fig. 4), which, besides emphasizing the bilateral symmetry of this part, serve respec- 

 tively to mark off the maxillary and thoracic regions, thus leaving the abdominal region con- 

 spicuous from the absence of any such differentiation of its surface. The general shape 

 of the folds may be compared to a figure made up of the Arabic numeral 3 for the right 

 fold and the same figure reversed for the left fold. The anterior por- 

 tions of both folds are thicker and approach each other closer than the 

 posterior portions. The folds are thickest and most sharply defined in 

 the maxillary region. At their origin, apparently within the bounds of 

 the head folds, they arise sharply from the general surface of the embryo Fi g S ' 2] ' 0utIi 1j e of 

 gradually increasing in breadth until near the middle of their length, the head K e . g . ion t. a 

 and then as gradually decrease until they pass into the thoracic folds. If atotfo<*iY 1 ^k2d! 

 The latter are of the same breadth throughout their extent and x a 5< 0utiine of a trans- 

 fade insensibly into the surface of the abdominal region. head\ttMs'°tem f x6o 



