Earlj Development of tlie Hen's Egg. 115 



scry cleavage, except in a few cases, is extremely difficult to detect 

 in the living egg; and it is only after the use of Flemming's fluid 

 that it becomes clearly demonstrable. Its difficulty of demonstra- 

 tion is further increased by the fact that the furrows most often occur 

 in radial planes, thus leading the observer to believe that they are 

 only the terminal ends of the primary cleavage furrows. We have 

 been unable to find a satisfactory explanation as to why the clefts 

 take this radial direction, unless -it be that they follow the course 

 of least resistance. I^ot all of the accessory cleavages, however, 

 have their furrows lying in radial planes, for the writer has found 

 several cases in which they were lying in various planes; and, fur- 

 thermore, he has observed two very clear cases of completely formed 

 accessory cleavage cells (Figs. 13 and 17). 



The contention of Blount that the accessory cleavages in the 

 pigeon's egg completely disappear and, therefore, these cells take 

 no part in the formation of the embryo, receives a full confirmation 

 from these studies. The demonstration of that conclusion is even 

 clearer in the hen's egg than in the pigeon's egg. In the hen's egg 

 the marginal cells never become closed, thus indicating that their 

 closing in the pigeon's egg must be in response to a stimulus re- 

 ceived from the numerous accessory cleavage cells. Probably the 

 closed margin cuts off influences which emanate from the accessory 

 cells, and which might interfere with the normal development of the 

 blastoderm. 



At best the accessory cleavage in the hen's egg is but a weak 

 attempt at cell formation, which has become inhibited, shortly after 

 its initiation, by the degenerative tendency of the accompanying 

 nuclei. 



IX. Fifteen to Seventeen Cells. — ^Foue and One-half Houks. 



In the stage consisting of approximately sixteen cells there are 

 four or five central and eleven or twelve marginal ones (Figs. 17 and 

 20). The central cells increase by the cutting off of the inner ends 

 of the marginal cells, while the latter multiply by the formation of 

 radial furrows. In some cases one can still see a tendency in the 

 form of the cleavage to remain regular. In Fig. 20 the anterior 



