EARLY DEVELOPMENT OF THE PIGEON'S EGG. 235 



The cells of this section will be recognized as the cells with the 

 .corresponding letters in Fig. 2. The small cell b is cut off from 

 the underlying yolk in this section, but it is so only at its central 

 end. In sections anterior to this, the cell b is continuous with 

 the yolk. The split made by this horizontal cleavage marks the 

 position of the future segmentation cavity. This horizontal plane 

 may not be permanently established at this stage. It seems to 

 come and go during the next few hours of development. But its 

 position indicates the depth of the center of the blastoderm in cleavage 

 stages. 



A comparison of the several stages here represented (Figs. 3, 

 5, 6 and 9) by actual measurement of the drawings will show 

 that there is but the slightest variation in the depth of the germi- 

 nal disc at the center. 



In contrast with this, is the account of the hen's egg by 

 Kolliker (6). He describes the blastodisc as increasing in depth 

 as cleavage progresses. In his Fig. 19 (which represents a ver- 

 tical section through a hen's egg of about twenty cells) two 

 central " Furchungskngeln" and two marginal " Segmental" are 

 shown ; i. e., there are four cells in the section forming a single 

 layer. Between this layer of cells and the white yolk is the 

 unsegmented " Bildungsdotter." None of these products of 

 cleavage is completely cut off from the " Bildungsdotter." They 

 form a layer .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 Fig. 22 where, " die Dicke der durchfurchten Stelle in 

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

 fruiter beschreibenen Fdlle {Fig. 1 9) ndmlich 0.28-o.jo mm." . . . 

 " Somit greift die Durchfurchung, 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. 19 mit bd als ungefurchten 

 Bildungsdotter bezeichnet ist." 



Kolliker suggests that the adding of cells from below may be 

 by a process similar to the adding of cells to the central part 

 from the marginal segments, — i. e., the nucleus of a marginal 

 segment divides and the central end of the segment containing 

 one of the daughter nuclei is cut off and becomes a " Furchungs- 



