46 Mary Blount. 



from being identical with Harjier's ('04) Fig. 43, and these both 

 resemble the eight-celled stage of the teleost (Wilson, II. V., '91, 

 Fig. G, and Agassiz and Whitman, '89, PI. xx. Fig. 19). Fig. 30 

 is a diagram of the teleost egg in the eight-celled stage. It is num- 

 bered for comparison with certain pigeon eggs. Fig. 27, 28, 29. The 

 numbers merely indicate the homologies of the cells and are not 

 intended to signify the order of segmentation. 



Following the eight-celled stage, cells are cut off from the central 

 ends of the large blastomeres and there are then established three 

 principal regions, as I have described in my preliminary paper: 

 (1) the central area, (2) the marginal cells, (3) the periblast. 

 The cells of the central area are Kolliker's "Furchungskugeln." The 

 marginal cells are his ''Segmenten." As cleavage proceeds, cells are 

 cut off centrally from the marginal cells and added to the central 

 area, and thus the latter grows at the expense of the former. Radial 

 cleavage planes divide the marginal cells, and increase their number 

 while the central cells are constantly becoming smaller by division. 

 This is well illustrated in photographs of a number of different 

 eggs (Figs. 46 to 54) and especially in two photographs of the same 

 egg (358), (Figs. 50 and 51). 



Finally, the marginal cells are all used up and we recognize 

 only two regions in the blastoderm: (1) the central area, and (2) the 

 periblast. In early stages, all of the cells are continuous with the 

 yolk (see Fig. 4), but as development proceeds the central cells 

 become complete below and separate from the yolk (see Figs. 5, 6, 

 11) and only the marginal cells are open below. Thus the marginal 

 cells constitute a "zone of junction" (see Agassiz and Wliitman, '84, 

 Figs. 2, 3, 4, and 5) between the segmented and unsegmented parts 

 of the egg. 



V. The Segmentation Cavity. 

 Description by Duval. — The position of the segmentation cavity 

 in the bird's egg has been a subject of considerable discussion. 

 Duval ('84) gives two figures (Figs. 4 and 5) of longitudinal sec- 

 tions of just laid, unfertilized hen's eggs, in which he identifies 

 the segmentation cavity as a small space just below the upper layer 



