90 GEORGO ORIHAY SHINJI 



5. ESTABLISHMENT OF THE EXTERNAL FORM OF THE EMBRYO 



The development of the embryo was traced up to the com- 

 pletion of the blastoderm as shown in figure 80. I first describe 

 the development of the embryo as seen mostly from surface 

 views. 



The first change externally visible after the completion of the 

 blastoderm is a depression or an invagination near the posterior 

 end of the egg. It is, at first, very shallow, but gradually 

 deepens forming a V- or U-shaped structure (fig. 45). This 

 condition is much more pronounced in the case of the cottony 

 cushion scale than in the other two species studied. The portion 

 of the blastodermic layer constituting the bottom of the blasto- 

 pore and its near-by area increases greatly in thickness, while 

 toward the anterior pole and in the area surrounding the blasto- 

 pore it becomes thin. In the cottony cushion scale, the colony 

 of parasitic organisms, originally found at the posterior pole of 

 the egg, is later pushed, so to speak, toward the anterior pole 

 by the elongation of the invaginating germ band.- During the 

 same period of development, the mass of the parasitic organisms 

 in the eggs of Pseudococcus and of Lecaniodiaspis has also 

 migrated a short distance from its point of entrance, the anterior 

 pole, towards the posterior pole. A side view of a similar embryo 

 (fig. 52) shows that the invagination occurs, not exactly at the 

 posterior pole, but a short distance lateral to it. The same 

 figure also brings out the fact that the layer forming this de- 

 pression and also the ventral germ band is essentially a con- 

 tinuation of the outer blastoderm, a portion of which forms the 

 serosa. Only the lower, or the ventral portion of these, later 

 becomes the embryo, while the upper or the dorsal wall trans- 

 forms into the amnion. A later condition of the egg is repre- 

 sented in figures 59 and 62. The invaginating germ band is 

 now about two-thirds as long as the egg. In the side \dew of a 

 slightly older embryo the germ band appears as though it con- 



2 In this paper the term germ band is used to designate the ventral wall of the 

 invaginated embrj'onal rudiment in its early stage. The term ventral plate 

 denotes the same structure after the amnionic layer becomes clearly distinguish- 

 able as the dorsal wall of the invagination. 



