118 GEORGO ORIHAY SHINJI 



comparable to those of Coleoptera and Diptera, yet, even in this 

 case, the germ cells arise just about the same time as those of 

 Coleoptera. The facts just mentioned, that the germ cells of 

 coccids, especially of Pseudococcus, become distinguishable as 

 such long before their approach to the colony of parasites, in- 

 dicate that the germ cells are by some chemotaxic action attracted 

 to the inclusion and do not become germ cells because they come 

 in contact with the latter. 



10. THE FORMATION OF THE DIGESTIVE TRACT 



The digestive tract of the embryo of the scale insects consists, 

 as in other insects, of the fore-, mid-, and hind-gut. The procto- 

 daeum and stomodseum in the scale insects are formed in exactly 

 the same manner as in other insects. The stomodseum grows 

 from its ventral opening, the mouth, dorsally, traversing the 

 future nervous rudiment almost perpendicularly, until it meets 

 the long axis of the egg, and then it turns posteriorly and meets 

 with the midgut rudiment. The posterior (proctodseum) invag- 

 ination, on the other hand, takes place on the tenth abdominal 

 segment, the segment which, at the time when the anus is exter- 

 nally visible, still lies curled over the thoracic segment (fig. 50). 

 From this time on until the embryo completes its revolution, 

 the posterior invagination proceeds very slowly, as a comparative 

 study of the figures 126 and 127 will show. However, the rate 

 of invagination after the revolution is exceedingly rapid. 



The midgut of Pseudococcus macdanieli Hollinger (MS) arises 

 from the entodermal cells, the origin of which has already been 

 described. 



The condition of the entodermal cells, at the time the thoracic 

 appendages begin to have two segments, is shown in figures 93 

 to 96. These represent consecutive longitudinal sections of the 

 same embryo. The purpose of giving these serial sections is to 

 show that the cells forming the entoderm are not in any way 

 connected with the cells surrounding the colony of symbiotic 

 organisms; that they are an altogether different kind of cells; 

 that they are clearly separated from the mesodermal layer, and 

 that they are also distinct from the ectodermal cells. The 



