EMBRYOLOGY OF COCCIDS 119 



entodermal cells, as illustrated in these figures, constitute two 

 parallel layers extending from the caudal extremity, where the 

 three germ layers arise, cephalad as far as the sixth or seventh 

 abdominal segment. The cells of the ectodermal layer at this 

 stage are ellipsoidal, and their nuclei stain exceedingly dark with 

 iron-alum haematoxylin, so they can be distinguished easily 

 from the spherical cells lying below them. The spherical cells, 

 which form a layer closely opposed to the ectoderm, are larger, 

 with richer nuclear contents than the cells of the ectoderm. 

 Thus it is clear that there is no possibility of confusing the 

 entoderm cells with the cells of the two other germ layers. 



The germ cells situated around the colony of symbiotic organ- 

 isms, however, resemble the entoderm cells in many respects. 

 The common origin of the germ cells and entoderm cells, however, 

 can be disregarded by the facts that they originate at the two 

 different periods in the development of the embryo and that, 

 at any time during their respective histories, the former never 

 migrate caudad beyond the third and the latter cephalad beyond 

 the seventh abdominal segments. 



Figures 126 and 128 represent two longitudinal sections of 

 different embryos, similar to those in figures 48 and 51. In both 

 cases the entodermal cells are much compacted near the posterior 

 end of the embryo. A crescentic groove is shown in figure 126. 

 This compacted condition of the entoderm and the appearance 

 of a clear groove within the entoderm cells are both due, not to 

 the actual massing or proliferation of cells as may be surmised, 

 but to the proctodaeal invagination which causes the elevation 

 of the posterior portion of the entodermal layer. 



Following these stages, the entoderm cells multiply rapidly 

 and form a coiled tube (fig. 129), so that a transverse section 

 through the fourth abdominal segment passes through the ento- 

 derm three times as shown in figure 113. The midgut- tube is 

 pushed still further in toward the cephalic end of the embryo 

 by further ingrowth of the proctodaeum until its anterior end 

 meets and unites with the posterior elongation of the stomodaeum, 

 the history of which has already been described. The condition 

 of the alimentary canal shortly after the union of the stomodaeum 

 with the midgut is shown in figure 14. During the stages 



