Early Development of the Hen's Egg. 121 



The periblast still remains free of nuclei, but the marginal cell 

 nuclei are beginning to show a tendency to migrate farther peripher- 

 ally than usual (Figs. 28 and 29). 



XIII. Three Hundred and Forty-six Cells in Surface View. — 



Eight Hours. 



The final stage that we shall consider in this paper is shown in 

 Fig. 32. During the hour intervening between this and the previous 

 stage the marginal cells have added many more cells to the central 

 area than at any former period of like duration, and consequently 

 the (radial) length of the marginal cells has greatly decreased, and 

 their furrows now are beginning to cut out into the periblast. 



This stage is one of the most important of all the early cleavages, 

 because it represents the transitional period between the "unorgan- 

 ized" and "organized" periblast; but we shall not consider the sec- 

 tions at this time. 



XIV. General Summary. 



The absence in this paper of comparisons between the develop- 

 ment of the hen's egg and that of other vertebrate eggs is not due 

 to a lack of appreciation of the importance of such comparisons, but 

 rather to the fact that in the main these have been pointed out for 

 the corresponding stages of the pigeon's egg. In this connection the 

 writer wishes, therefore, to confine himself to emphasizing the close 

 similarities between the development of the hen's egg and that of the 

 pigeon, although to those who have followed closely the work on the 

 latter egg this may seem unnecessary. 



Exact agreement in all details of development, even in the eggs 

 of two species as closely connected as those of the hen and the 

 pigeon, is not to be expected, but the fundamental processes should 

 certainly agree. And such has proved to be the case. The minor 

 differences in development of these two forms have to do primarily 

 with time relations ; for although the eggs of these two species are 

 in about the same stage of development at the time of laying, yet 

 the pigeon's is forty-one hours old and the other's twenty-two. The 

 holomogous processes, therefore, necessarily do not occur at exactly 

 the same time after fertilization. 



