Gastrulation in the Pigeon's Egg. 81 



At the posterior end of this same section (Fig. 3) an entirely 

 different condition is found. Aside from the thinness of the margin, 

 the almost entire disappearance of the zone of junction is the most 

 characteristic feature. The only remnants of it are the degenerating 

 periblastic nucleus {pn) and the single cell which is about to arise 

 from the yolk (ce). For a considerable distance on either side of 

 the posterior portion of this section the only normal periblastic nuclei 

 visible are the few w^hich are in the last stages of acquiring dis- 

 tinctly outlined cell limits. All other nuclei are in some phase of 

 degeneration. About twenty-five sections to either side of the median 

 line, however, the uninterrupted zone of junction is again found. 

 In Fig. 4, which is taken twenty sections to the right of the center, 

 two normal nuclei are forming cells about them (ce), and to the 

 left of these there is a completed cell. Two degenerating nuclei 

 are also present {pn). Five sections farther to the right, the first 

 indication of a true zone of junction is found (Fig. 5). The zone 

 here is very narrow, but still farther to the side it becomes much 

 w'ider (see Fig. VI). 



From this time on the thinning-out of the blastoderm and the 

 interruption of the posterior zone of junction make rapid progress, 

 until about thirty-one to thirty-three hours after fertilization, when 

 the zone is completely interrupted for a distance of 70-80 degrees 

 (Fig. VII) (cf Fig. V, F). Comparing Fig. 29 with that of a 

 longitudinal section of such a blastoderm (Fig. 30), it is apparent 

 that the condition of the latter has been brought about as the result 

 of processes already described in connection with the former, and 

 consequently, the section ends posteriorly in a thin free margin (Fig. 

 14), with the zone of junction entirely wanting. In passing for- 

 ward, however, one finds a gradual increase in the thickness of the 

 lilastoderm (Fig. 30). The subgerminal cavity, which has increased 

 l)otli in depth and extent, is occupied by many segmentation cells, 

 which, for the most part, lie in a row near the floor of the cavity. 

 The position of the cells is purely an artifact — a condition produced 

 during fixation ; for a study of this section under high power 

 reveals the fact that the upper contour of every nucleated cell or 

 group of cells lying in th^ cavity exactly corresponds to the under 



