DEVELOPMENT PRIOR TO LAYING 47 



but beyond the zone of the accessory cleavage there may occur 

 two or three concentric circles variously indicated (Fig, 17). 

 Vacuoles, appearing black in the photographs, are very common 

 in the outer zones. These appearances indicate that the peii- 

 blastic protoplasm extends farther out in the superficial white 

 yolk than is usually believed to be the case; and this suggests an 

 interesting comparison with the teleost ovum, where the peri- 

 blastic protoplasm surrounds the entire yolk as a very thin layer. 

 Sections confirm the idea that the periblastic protoplasm has an 

 extension beyond the so-called margin of the blastodisc. Some 

 eggs show a more definite margin than others; it may be that 

 there is a periodic heaping of the periblast at the margins, for 

 which again an analogy may be found in teleosts. 



Although the smallest cells may be more or less excentric in 

 the segmented germinal disc of the pigeon, their position bears 

 no constant relation to the future embryonic axis. They may 

 lie in this axis in front of or behind the middle, or to the right or 

 left of it (cf. Fig. 18 A-D). 



At the eight-celled stage a horizontal cleavage plane begins to 

 appear beneath the central cells (Fig. 19). This marks the full 

 depth of the blastoderm at all stages, and the several-layered 

 condition arises by horizontal cleavages between this and the 

 surface. Comparison of Figs. 19, 20, and 22, drawn at the same 

 magnification, will show that the depth does not increase by addi- 

 tion of cells cut off from below, as is usually supposed to be the 

 case in the bird's ovum. The first horizontal cleavage plane not "? 

 only marks the full depth of the blastoderm, but it also indicates ) 

 the site of the segmentation cavity which arises gradually by accu- 

 mulation of fluid between the cells and the underlying unseg-' 

 mented protoplasm and yolk. The segmentation cavity gradually 

 extends towards the margin of the blastoderm, but it is bounded 

 peripherally by the zone of junction between the marginal cells 

 and the periblast. 



IV. Origin of the Periblastic Nuclei, Formation of the 



Germ-wall 



Our knowledge of this part of the subject in the hen's egg is 

 very incomplete, and the various accounts are contradictory. 

 The reason for this is the great difficulty of securing a complete 

 series of stages, and of arranging them in proper sequence. There 



