STUDIES ON THE EAELY DEVELOPMENT OF THE 



HEN'S EGG. 



I. History of the Eakly Cleavage and of the Accessory 



Cleavage/ 



J. THOMAS PATTERSON. 

 With 32 Figures. 



I. Introduction. 



The period extending from ovulation to the laying of the egg 

 is a most obvious gap in our knowledge of the development of the 

 hen's egg. It has been the writer's desire to fill in this break, and 

 he is indebted to the trustees of the ''Elizabeth Thompson Science 

 Eund" for a gTant which made it possible to undertake the work. If 

 the problem contained no possibilities other than that of merely 

 filling in a gap, it is doubtful whether the work would have been 

 undertaken, since the results could not have been commensurate 

 with the labor involved. But it was felt that certain points, brought 

 out in a study of the pigeon's egg by several students at the Uni- 

 versity of Chicago (Harper, '04; Blount, '09; Patterson, '09), 

 deserved further investigation. Among these were fertilization, ac- 

 cessory cleavage, and gastrulation. 



On account of the importance centering in gastrulation and the 

 accessory cleavage, their discovery in the hen's egg would be of 

 the greatest interest ; for a true gastrulation has never been found in 

 this egg, and the accessory cleavage has been neither figured nor 

 described. We have not even known whether fertilization in the 

 hen's egg is monospermic or polyspermic. 



^Contribution from the Zoological Laboratory of the University of Texas, 

 No. 103. 



The Journal of MoKPHOLOGy. — Vol. 21, No. 1. 



