48 Mary Blount. 



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 "Furchungskugel." The other 

 daughter nucleus passes into the marginal segment, and so on until 

 finally the part of the marginal segment left over changes into a 

 "Furchungskugel." And so, according to Kolliker, the first appear- 

 ing "Furchungskugeln" are never completely cut off from the unseg- 

 mented "Bildungsdotter" below, but nuclei, sisters to those in the first 

 layer of cells, pass down into the "Bildungsdotter." Here nuclear 

 division takes place, and cells are organized around the upper 

 daughter nuclei, thus forming the second layer of cells in the center 

 of the blastodisc, while the lower daughter nuclei are left deeper 



wd 



Fig. 42.— Copy of Kolliker's ('76) Fig. 22. Senkrecliter Schnitt durcb die 

 Furcbungsstelle eines Hiihiiereies aus dem Uterus, s, grosses Segment; s', 

 kleines Segment ; k, grosse einschichtige Randkugeln ; k', kleinere Kugelu aus 

 der Mitte geschichtet ; wd, weisser Dotter. 



in the "Biildungsdotter." And thus cleavage proceeds downward 

 until finally the last remaining nucleated portions of the "Bildungs- 

 dotter" change over into "Furchungskugeln." 



The Segmentation Cavity of the Pigeon's Egg is homologous with 

 that of other vertebrate eggs. — But in the pigeon's egg, I do not 

 find any such deepening of the center of the blastodisc. The change 

 from one to several layers of cells is by a process exactly like that 

 of the teleost egg. (See Agassiz and Whitman, '84, Fig. 2, and 

 Wilson, H. W.-, '91, Figs. 16, 17, 18, and 19.) The blastodisc of 

 the pigeon's egg becomes stratified by horizontal cleavage planes 

 arising above the first horizontal cleavage, i. e., above the level of 

 the plane which limits the cell h below (Fig. 5). Nuclei are never 

 found in the central part below the level of the horizontal cleavage 

 under tlie cell h, — at least not until a much later stage. 



