46 THE UNIVERSITY OF OKLAHOMA 



average of four were observed to be in the process of division. Of 

 the separate germ layers, at this stage the mesoderm shows the 

 highest rate with a mitotic index of 5.0208, the ectoderm follows 

 with 4.5509, while the entoderm shows the lowest rate (2.494). 

 The cells of this chick are characterized by their relatively large 

 nuclei. 



33-Hour Chick 



In the 3,:' honr chick where differentiation is taking place there 

 lias been a rapid decline in the rate of cell multiplication, the aver- 

 age mitotic index lieing 2.156 compared with 4.139 in the 18-20 hour 

 stage. Here, the ectoderm (2.339) shows the highest rate of growth, 

 the mesoderm (2.072) follows with the entoderm (1.952) showing 

 the lowest rate. 



48-Hour Chick 



The 48 hour chick shows a higher rate of cell division than the 

 33 hour stage but less than the 18-20 hour chick, the mitotic index 

 here being 3.121. In this stage the preponderance of the head is a 

 marked feature, the forebrain being bent at right angles to the 

 axis of the embryo. This extraordinary development of the cen- 

 tral nervous system together with the resultant extra growth of the 

 ectodermal covering explains the increase in the rate of division of 

 the ectoderm of this chick over that of the 33 hour stage. The 

 mesoderm and entoderm show a much smaller rate of increase. The 

 mitotic index for ectoderm here is 3.896, mesoderm 2.768. and ento- 

 derm 2.611. 



72-Hour Chick 



The 72 hour chick, where differentiation takes place on a larger 

 scale shows the slowest rate of growth of all the chicks studied, the 

 mitotic index being 1.736. The ectoderm here in 2.805, mesoderm 

 .8708 and entoderm .914. Here we notice the parts are growing 

 rather irregularly, some faster, some lower, although on the 

 whole the rate is slower. 



General Conclusions 



The rate of cell multiplication as shown by my table of totals 

 proves that in all stages of the chicks that I studied, the anterior 

 end in general exceeds the rest of the chick in its rate of cell 

 multiplication, the tail region comes next, while the slowest rate 

 occurs in the central part that is the region of the yolk sac. Be- 

 cause of this higher rate of growth differentiation manifests itself 

 in this head region first with the tail and yolk sac regions follow- 

 ing in their respective order. 



My tables also show the ectoderm in all the stages studied 

 except the 18-20 hour has the highest percentage of cell multi- 



