494 BERTRAM G. SMITH 



it somewhat resembles a blastopore (fig. 221). Later, the groove 

 becomes faint at its posterior margin, very pronounced at its 

 antero-ventral margin (fig. 222) . The area enclosed by the groove 

 diminishes in size with its forward movement; it also becomes 

 almost transparent. Since throughout the remainder of its 

 history this area strikingly resembles a window, I shall refer to 

 it as the fenestra. 



In material fixed in a modification of the bichromate-acetic- 

 formalin mixture (see Smith '12, Section III, Solution B) con- 

 taining twice the usual amount of potassium bichromate, the 

 fenestra is cut up into small polygonal areas separated by furrows 

 that greatly resemble cleavage furrows (figs. 116 to 120 and 222; 

 cf. Ishikawa, '08 and '09). These polygonal areas do not repre- 

 sent single cells; each comprises a group of several cells. The 

 phenomenon is not entirely an artifact, since it often appears, 

 though faintly, in the living egg. By this method of fixation 

 the septal furrow is likewise accentuated. 



Before describing the further history of the fenestra it is desir- 

 able to direct attention to some other changes in the upper hemis- 

 phere as observed in the living egg. 



About the time that the fenestra becomes limited to the ante- 

 rior half of the upper hemisphere by the upgrowth of the poste- 

 rior margin of the opaque region, a translucent area, the roof 

 of the gastrocoele, appears in this region of upgrowth (figs. 125 

 and 126). This translucent area is at first crescent-shaped; it 

 is separated from the more transparent fenestra by an opaque 

 band which is the outward expression of the septum separating 

 the gastrocoele from the blastocoele. 



As soon as the septum has advanced into the anterior half of 

 the upper hemisphere, the translucency of the roof of the gas- 

 trocoele extends backward almost to the blastopore — evidently 

 by the deepening of the gastrocoele in this region, admitting 

 light. Meanwhile each postero-lateral margin of this region 

 becomes bordered with a faint band of a slightly more opaque 

 character — an effect due largely to the early mesoderm (figs. 

 127 to 129), though the entoderm is also concerned in producing 

 it. 



