58 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



outer surface of the serosa. These consist of a very thin yellow cuticular 

 layer, or cuticuHn, and a white chitin-like cuticular layer which is within 

 the first, also secreted by the serosa. The white cuticle shortly before 

 emergence is dissolved by an enzyme; the yellow cuticle persists unchanged 

 to the end. According to Slifer the enzyme noted above is secreted by 

 the pleuropodia. In the yellow cuticle of Melanoplus there is a small, 

 circular, specialized area through which water enters or leaves the egg 

 and which has been called by Slifer (1938) the "hydropyle." Wheeler 

 has called attention to a similar cuticle secreted from the surface of the 

 serosa and of the indusium in Xiphidium. The cuticle which is molted 

 during the embryonic life of the centipede and the crenated membranes 

 which are cast off by the embryo of Isotoma present analogous cases. 



THE PRIMARY DORSAL ORGAN 



Attention has already been called to the primary dorsal organ, a 

 specialized structure which is most highly developed in the Collembola. 

 As has been stated, although some of the earlier writers regarded both 

 the primary and the secondary dorsal organs as the homologues of the 

 serosa, more recently it has been shown that the primary organ differs 

 from the secondary organ in structure, origin, function, and time of 

 appearance. In Isotoma it appears early at the anterior pole of the egg 

 immediately after the formation of the inner layer and the appearance 

 of the germ cells. Cells from the anterior pointed end of the egg become 

 differentiated from the adjacent epithelial cells. As they become deeper, 

 the cells of the inner layer immediately below are dislodged and disappear 

 in the yolk. As development proceeds, the cells penetrate more deeply 

 into the yolk, progressively changing in appearance (Fig. 75), the cluster 

 assuming the character of a gland. The organ in Isotoma remains a 

 conspicuous structure until after the revolution of the embryo, when it 

 rather suddenly disappears entirely. To judge from its appearance there 

 can be little doubt of its glandular nature. Similar structures have been 

 described for Anurida and Podura. 



In beetles a vestigial structure has been described that may be 

 homologous with the organ just mentioned. Hirschler (1909) found a 

 structure in the early embryo of the chrysomelid beetle Donacia that 

 consists of an oval dorsad-lying area of primary epithelium which sinks 

 down into the yolk and becomes covered by the epithelium. Its com- 

 ponent cells meanwhile send out processes into the yolk. This structure, 

 designated by Hirschler as the "primary dorsal organ," finally degener- 

 ates completely and is absorbed in the yolk. 



In the honeybee, Nelson (1915) has described what he termed the 

 "cephalic-dorsal body," which in position, overgrowth by the primary 

 epithelium, and final absorption in the yolk closely corresponds to the 



