98 GEORGO ORIHAY SHINJI 



stage of development here illustrated that the mesodermal cells 

 become easily distinguishable from the other two cell layers, 

 namely, the columnar ectodermal cells forming the floor of the 

 invagination above and the layer of much smaller and feebly 

 staining syncytial cells below. The mesodermal cells at this 

 time can be distinguished from the ectodermal cells by their 

 large size and coarsely granular nuclear contents. The staining 

 reaction of the two is also different : the mesoderm stains heavily, 

 the entoderm only very feebly. 



Up to this stage, invagination, in the case of Pseudococcus, 

 is indicated by a slight depression at a short distance ventrad 

 of the posterior pole of the egg. As the invagination progresses 

 further, the depression becomes deeper and its walls more closely 

 compressed (fig. 85). Mention should be made of the fact that 

 the depression in Icerya and perhaps in case of the other species 

 is, at first, in the form of a cup with broadly rounded bottom. 

 The entodermal cells are always found opposite the base of this 

 cup-like depression. With the further growth of the embryo, 

 the dorsal wall of the depression, now a compressed tube, be- 

 comes thinner and, at the time when the embryo reaches the 

 anterior mass of the parasitic organisms, it becomes reduced to 

 a layer one cell thick. The ectoderni cells also undergo a marked 

 change. They are arranged in a layer beneath or ventrad to the 

 amnion, and, at the same time, become much more elongated, 

 with their nuclei arranged alternately, giving the appearance of 

 two layers. The mesoderm, again, has spread out to its full 

 length below the ectoderm, which is now the ventral plate. The 

 cells of the ectoderm, mesoderm, and entoderm are still con- 

 tinuous at the extreme bottom of the invagination where the 

 primitive condition still exists. 



From this time on, the caudal or abdominal portion of the 

 embryo elongates, extending over and above the ventral surface 

 of the ventral plate. Consequently, any transverse section 

 passing through the thoracic region also passes through an 

 abdominal segment. Figure 103 shows such a section through 

 the second maxillary segment and one of the abdominal segments. 

 Here, in the ventral or thoracic section, the amnion appears as a 



