The Early Development of the Pigeon's Egg. 



47 



of cells. Below the segmentation cavity there is a layer of cells 

 continuous with the yolk. Stages of later cleavage of the pheasant 

 and the canary are shown in Duval's Figs. 7 and 8 respectively. 

 Here again the segmentation cavity is below the upper layer of 

 cells, i. e., it is between the upper layer and a mass of cells which has 

 been added to the germinal area from the syncytial yolk. 



Description hy Kolliher. — Kolliker ('76) describes the blastoderm 

 of the hen's egg as increasing in depth by additions from the 

 "Bildungsdotter." His Fig. 18 presents the surface view of the 

 germ area of a hen's egg in which there are twenty-one cells, and his 

 Fig. 19 gives a vertical section through the same egg. This section 

 shows one layer of four cells in which there are two central 

 "Furchungskugeln" and the two marginal "Segmenten." Below this 

 layer of cells is the unsegmented "Bildungsdotter" and below that 

 is the funnel-shaped white yolk (Fig. 41). He describes the blasto- 

 derm as increasing in depth as cleavage progresses. None of the 



Fig. 41. — Copy of Kolliker's ('76) Fig. 19. gd.. Gelber Dotter. wd., weisser 

 Dotter. bd., ungefiircliter Bildungsdotter. s', grosses Segment, s, kleines 

 Segment, k, Kugeln. 



cells is completely cut off from the "Bildungsdotter." They fonn 

 a layer 0.14 mm. in depth in the center. A section through a later 

 stage in the development of the hen's egg is shown in Kolliker's 

 Figure 22, (Fig. 42) where "die Dicke der durchfurchten Stelle 

 in der Mitte des Keimes gerade noch einmal so dick war, als in dem 

 friiher beschriebenen Falle (Fig. 19 ), namlich 0.28-0.30 mm." 

 "Somit greift die Durchfu.rchung indem sie weiterschreitet in der 

 Mitte der Keimschicht immer mehr in die Tiefe, wie schon Oellacher 

 dies vermuthet hat, und erreicht am Ende nahezu die Grenze der 

 Lage die in der Fig. 10 mit bd als ungefurchter Bildungsdotter 

 bezeichnet ist." (Kolliker, '76, p. 74.) 



