DEVELOPMENT OF PARAVORTEX GEMELLIPARA 523 



portion was absorbed by primary entoderm cells which were in 

 turn ingested by amoeboid secondary entoderm elements. The 

 ectoderm grew backward over the entoderm and at the same 

 time absorbed the remainder of the vitellarial yolk. Figure 23 

 was pointed to as illustrating the appearance of the embryo at 

 the close of gastrulation . By far the greater portion of the 

 embryo is occupied by the yolk-laden cells. 



The fate of this yolk now remains to be described. In the 

 stage represented by figure 23 little change has occured in the 

 nutritive material. It is of the same nature as the free yolk 

 seen in figure 21, with the exception that the flakes and globules 

 are less closely packed in the cytoplasm of their containing cells. 

 There is observable, however, a tendency of these particles to 

 flow together into larger globules, a process which is more rapid 

 in the ectoderm cells. Thus in the two epithelium cells at the 

 top of figure 23 several such spheres are conspicuous. 



In an older embryo (fig. 34) this sphere formation has advanced 

 considerably. The spherules of nutritive substance combine 

 again and again until the major portion is included in one or 

 two immense globules. That the yolk is undergoing a trans- 

 formation is indicated by the appearance of darker spherules 

 suspended in the large masses. These smaller elements stain 

 dark brown or black in iron-haematoxylin preparations such as 

 figure 34 represents. 



In figure 25 is illustrated the appearance of the ectodermic 

 nutritive material in the living embryo after treatment with the 

 neutral red stain. The latter has a more rapid effect upon the 

 larger masses than upon the smaller. After immersion for a 

 veiy few minutes in a dilute neutral red solution it is impossible 

 to see through the posterior ectoderm; all is a mass of deeply 

 stained red spheres. 



Returning to figure 34 it is seen that clear regions have appeared 

 in the interior of each entodermic yolk cell. These regions in 

 the iron-haematoxylin-eosin preparations are perfectly colorless, 

 but in those treated with Ehrlich's haematoxylin and eosin 

 after fixation with corrosive acetic, they take the eosin color 

 (figs, 27, 36). Thus it is evident that these lighter portions are 



