DEVELOPMENT PRIOR TO LAYING 43 



Duval interprets a narrow space observed by him between the single 

 superficial layer of cells and the deeper cells of the germinal disc as the 

 segmentation cavity; it is thus entirely distinct from the subgerminal 

 cavity which arises much later, according to his conception. Apart 

 from the fact that his figures appear to represent the merely virtual 

 space between the superficial cells and the underlying cells in an exag- 

 gerated form, the interpretation appears to me to be incorrect. It is 

 based on the theory that the deeper cells represent the primary entoderm, 

 a view which I cannot accept ; the interpretation of this space as cleavage 

 cavity fails if it be shown (see beyond) that the underlying cells are not 

 entoderm. 



The account given above of the deeper cleavages, those seen in 

 section, is the conventional one, based on the observations of Kolliker, 

 Duval, and others. The account, that follows, of the corresponding 

 cleavages in the pigeon's egg, is different in some important respects, 

 that bring it into agreement with the best known meroblastic eggs, 

 those of the bony fishes. I have, however, allowed the above account 

 to stand, though I consider it probable that a careful re-examination 

 would bring the cleavage of the hen's egg into line with that of the pigeon 

 and the teleost. 



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

 been worked out in more detail than that of the hen's egg (Blount) ; 

 as it offers some interesting features that have never been de- 

 scribed for the hen's egg, and must be made the basis of the 

 description of the formation of the germinal wall and the germ- 

 layers in the absence of any consistent account for the hen's egg, 

 it will next be described. The fundamental features of the cleav- 

 age are the same as in the hen's egg, so that the description 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 ISA). 

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

 around the supernumerary sperm-nuclei, which accumulate and 

 multiply in the periblast. The complete history of these nuclei 

 has been worked out by 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 



