THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 5 



the surface by the union of the amnion folds, and had come to lie in the yolk : Figure 

 V represents a transverse section of the egg at this stage with the embryo (E in 

 Fig. ) lying in the centre of the yolk. The embryo at this stage, as shown by study- 

 ing a series of sections, is watch-glass shaped, with its convex ventral side covered 

 by the amnion, which it will be remembered is the inner limb of the folds described, 

 and is constricted off from the surface of the egg with the embryo. In Figure V 

 the amnion covering the ventral surface of the embryo is not represented, but on re- 

 ferring to Figure VII, which is a cross section of an older embryo, the structure 

 and relation of the amnion (AM in Fig. VII) covering the ventral surface of the 

 embryo will be understood. The early watch-glass shaped embryo (Fig. V) consists 

 of a single layer of elongated epithelial cells with nuclei in some cases in process of 

 division on their ventral extremities, i.e. on the extremities at the convex or ventral 

 side of the embryo. A few nuclei with aggregated protoplasm about them lie on the 

 concave side of the embryo (YC Fig. V). These nuclei with the protoplasm about 

 them are probably cells about to migrate from the embryo into the yolk. 



In the earliest stages it will be remembered that the embryo was more than one 

 cell thick in some places (Fig. IV). Probably some of the cells of this early embryo 

 remaining on its dorsal or concave surface are the cells just described as about to 

 migrate from it into the yolk. Before the embryo reaches the stage represented in 

 Figure V the yolk undergoes segmentation. The segmentation of the yolk is 

 probably due to the multiplication of yolk cells. Radial processes from the pro- 

 toplasm of these cells adhere to a number of yolk spherules. Consequently there 

 is formed a structure known as the yolk ball or yolk segment, consisting of a central 

 cell, radial protoplasmic processes of which hold yolk spherules in position. YB 

 Figure XI represent a cross section of a yolk ball. Similar cross sections will be 

 observed in other figures. Adjacent yolk balls appear in some cases to be united; 

 some are connected with the blastoderm and others with the amnion or embryo, 

 consequently the entire egg is probably a protoplasmic continuum with all its parts 

 in connection. The structure of the blastoderm cells is shown in Figure VII. They 

 are flat hexagonal cells with conspicuous nuclei. On the union of the amniotic 

 folds, the embryo with the true amnion is constricted off from the surface of the egg 

 as described. With the sinking of the embryo into the centre of the egg the yolk 



