DEVELOPMENT PRIOR TO LAYING 43 



the segmentation cavity which furnishes a lower boundary to the 

 central cells. In later stages it extends peripherally to the inner 

 margin of the periblast, and thus all of the central cells become 

 completely bounded. A new class of cleavage planes then forms 

 in these cells after the thirty-two-celled stage, horizontal or parallel 

 to the surface; in this way the central part of the blastoderm 

 becomes two cell-layers deep, and later several layers deep. The' 

 segmentation cavity never cuts under the marginal cells, which 

 remain united below and at their margins by the periblast (Fig. 

 16 A). 



In the older accounts of the horizontal cleavages by Kolliker, Duval 

 and others these are represented as forming before the segmentation 

 cavity, thus leaving the deeper cell in continuity ^\ith the yolk. Such 

 cells are then supposed to continue budding off cells from their upper 

 surfaces. But this view has been shown to be incorrect by the observa- 

 tions of ^Miss Blount on the pigeon described below and by Patterson on 

 the hen included above. 



The Pigeon*s Egg. The cleavage of the pigeon's egg has 

 been worked out in detail by Miss Blount; as it must be made the 

 basis of the description of the formation of the germinal wall and 

 the germ-layers in the absence of anj^ consistent account for the 

 hen's egg, it will next be described. The fundamental features of 

 the cleavage are the same as in the hen's egg, so that the descrip- 

 tion need not be repeated. 



The feature to be particularly emphasized in the cleavage 

 of the pigeon's egg is the occurrence of a secondary or accessory 

 cleavage in the marginal zone or periblast (Figs. 17 and 18 A). 

 When the origin of these cells is traced it is found that they arise 

 around the supernumerary sperm-nuclei, which accumulate and 

 multiply in the periblast. The complete history of these nuclei 

 has been worked out ])y Harper and Blount, so that there 

 can be no doubt as to their derivation. Another interesting 

 point illustrated by the figures is that the marginal cells have 

 a peripheral wall wherever the accessory cleavage occurs, but 

 between the groups of accessory cleavage cells the marginal cells 

 are continuous with the periblast (Figs. 17 and 18 A,) as they are 



