238 MARY BLOUNT. 



As cleavage proceeds cells are cut off centrally from the mar- 

 ginal cells, and added to the central area, and thus the latter 

 grows at the expense of the former (compare Fig. 7). Radial 

 cleavage planes divide the marginal cells and increase their num- 

 ber, while the central cells are constantly becoming smaller by 

 division. 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, but as development proceeds, the cen- 

 tral cells become complete below and separate from the yolk, and 

 only the marginal cells are open below. Thus the marginal cells 

 constitute a "zone of junction" (see Agassiz and Whitman (1), 

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

 parts of the egg. All of the photographs presented in this paper 

 show a very symmetrical form of cleavage, and while I have 

 found a good many instances of asymmetrical cleavage, I cannot 

 agree with Kolliker (6) that "Die Furchung geht immer asym- 

 metrisch vor sich, so dass ohne Ausnahme die eine Halfte der 

 Keimscheibe in der Zerkliiftung der anderen voran ist." 



4. The L ast Stage of the Multiplication of the Sperm Nuclei. — 

 In an egg obtained at 6:30 A. M., ten and a half hours after fer- 

 tilization, the sperm nuclei were very numerous. There is no 

 record of the exact number of cells of primary cleavage showing 



Fig. 5. Transverse section of a pigeon's egg at the end of the period of multiplica- 

 tion of the sperm nuclei. Egg taken 6.30 A. M., about 10 hours after fertilization 

 and 31 hours before laying. Note that all cells are still continuous with the yolk. 1. 

 Accessory cleavage around the sperm nuclei. 2. Marginal cells sharply separated 

 from the sperm nuclei. 3. Central cells. 4. Sperm nuclei. 



on the surface of this egg, but there were a few more than thirty- 

 two. The accessory cleavage was very abundant and more than 

 one cell in depth. A transverse section through about the cen- 



