Early Development of the lien's Egg. 113 



early blastomeres to flow together, it may be that the original relation- 

 ship between the cleavage planes is modified. If this is not the case, 

 then the variation in the form of cleavage which characterizes the 

 later stages is anticipated in the eight-celled stage. 



As in the two- and four-, the cells of the eight-celled stage are 

 not true cells, in the sense that they are not completely delimited 

 by cell walls, but are open to the periblast both below and peripher- 

 ally. Occasionally, however, one of the blastomeres may be sur- 

 rounded (in surface view) by cell walls (Fig. 13). We have, there- 

 fore, two regions of cleavage, in which the cells are usually desig- 

 nated as central and marginal. 



The connections between both the central and marginal cells with 

 the periblast are very clearly brought out in the section (Fig. 18), 

 which also gives one a clear idea of the beginning of the horizontal 

 cleavage planes. These planes not only separate the blastomeres 

 from the underlying or central periblast, but also mark the position 

 of the future segmentation cavity. Such an interpretation for the 

 origin of the cleavage cavity is not in accord with the account of 

 Duval and others. Duval contends that a very narrow space situ- 

 ated between a single superficial layer of cells and the deeper cells 

 represents the segmentation cavity, and, furthermore, that the deeper 

 cells are derivatives of the deeper parts of the disc, and have arisen 

 by additions upward to the parts already segmented. Duval's view 

 has been shown Vo be untenable for the pigeon's egg, and I find it 

 is also incorrect for the hen's egg; but exactly the same thing occurs 

 in the latter egg as described for the pigeon's egg by Miss Blount. 

 The increase in the number of cell layers in the disc is not brought 

 about by the addition upward of cells from the underlying material, 

 but by the appearance of horizontal cleavages, which occur between 

 the segmentation cavity and the surface of the blastodisc and thus 

 the large central cells are cut up into a number of cell layers. 



The accessory cleavage remains distinct up to the eight-celled 

 stage (Figs. 14, 16, 13), and in the reconstruction of a series from 

 the seven-celled stage there are shown ten supernumerary sperm 

 nuclei (Fig. 14). Seven of the nuclei can be arranged into three 

 groups, indicating that originally but five sperms entered the egg 

 and not ten. Four of the nuclei are in the last stages of degenera- 



