EMBKYOLOGY OF COCCIDS 99 



thin membrane composed of a few flattened cells. The mesoderm 

 cells, which, on both sides, meet the amnion and ectoderm, form 

 a layer one cell in thickness. Although the lateral extensions 

 of the mesoderm have followed the evaginations of the append- 

 ages, the layer is still one cell in thickness. The dorsal or 

 caudal section presents nothing new. It resembles a section 

 through the thoracic region of a younger embryo. Both the 

 amnion and the mesoderm consist of spherical cells, while the 

 ectoderm cells are spindle-shaped. All three germ layers are 

 continuous at the sides. 



A longitudinal section of a somew^hat younger embryo of 

 Pseudococcus (fig. 99) and one of Lecaniodiaspis (fig. 89) show 

 that the amnion actually terminates at both the caudal and 

 cepahlic ends and that it incloses the parts of the embryo. The 

 layer of mesoderm, which is much thicker at the caudal region, 

 gradually thins out toward the oral region, the cephalic region 

 being entirely devoid of it. 



A transverse section of an older embryo (fig. 104) shows, 

 among other things, a remarkable change in the mesoderm. 

 On account of the neurogenic swellings, which occur on both 

 sides of the median line, the mesodermal cells have been pushed 

 aside, so to speak, from the area immediately dorsal to the 

 neurogenic area and have been shifted to the sides where the 

 evagination of the appendages has taken place. 



A transverse section through the second maxillae of a much 

 older embryo is shown in (fig. 107). The dorsal or caudal section 

 shows, among other things, that the amnion is represented by a 

 single layer of cells, and that the mesoderm has expanded, not 

 only into the evagination of the appendages, but also in the 

 dorsal extension of the ectoderm. 



A comparison of figures 103 and 107 will show that the dif- 

 ferentiation of germ layers takes place from in front backward, 

 as in other insects, except the agamic aphids, in which, according 

 to Will ('88), the cephalic portion develops earlier than the 

 brain. 



As the appendages further elongate, the mesoderm follows 

 the ectodermal evagination, forming a double-layered structure. 



