Early Development of the Hen's Egg. 117 



In median sections of the fifteen-celled stage the horizontal cleav- 

 ages have progressed to the point of effecting a complete separation 

 of the central cells from the central periblast (Fig. 22), and these 

 cells, are therefore, completely delimited by cell walls. The seg- 

 mentation cavity is expanding laterally beneath the marginal cells 

 by the extension of the horizontal clefts (Fig. 22, on left), and 

 v^ill eventually increase in depth by the accumulation of fluid 

 within it. 



X. Thirty-two to Thirty-five Cells. — Four and Three- 

 fourths Hours. 



In this stage (Fig. 23) the central cells have begun to multiply 

 more rapidly than those of the margin, and the two kinds are now 

 of equal numbers. The form of the cleavage is irregular, and, 

 therefore, very variable in different eggs. At the posterior border 

 of the blastoderm are seen two short radial furrows (Fig. 23, m), 

 and while it is possible that there were more at an earlier period, yet 

 probably this is an egg in which there were never many ; because if 

 numerous at an earlier period of development, the primary cleavage 

 furrows should give some evidence of having extended to the margin 

 of the primary area, and it will be noted that they fall quite short 

 of reaching the margin. 



The median section of a blastoderm, which showed thirty-two 

 cells (sixteen marginal and sixteen central) in the living egg, is seen 

 in Fig. 24, A. The cleavage cavity is especially well developed, and 

 the one-layered condition of the blastoderm is unusually clear. The 

 vertical cleavage separating the two cells which lie slightly to the left 

 (anterior) of the center has not completely cut through to the cavity. 

 Such a connection between adjacent cells is very slender, and, in 

 this particular case, is absent in a few sections lying to either side. 



While the planes of the early cleavages are, for the most part, 

 vertical (that is, meeting the surface of the blastoderm at right 

 angles), yet some of the planes take an oblique course. Under such 

 conditions it is not difficult to find many places where the blastoderm 

 appears to be two cells thick ; especially is this true of sections that 

 are taken some little distance to either side of the median line (Fig. 



