64 H. JEANETTE ALLEN. 



indebtedness to Dr. H. L. Wieman, under whose direction this 

 problem was undertaken, and whose assistance, interest and* 

 criticism made it possible to carry out the work. 



Perhaps the most striking fact observed in the formation of 

 glycogen in the chick was its almost invariable presence in the 

 yolk-sac, from the earliest stages to chicks of ten days. Fig. I 

 (Plate I.) shows globules of glycogen in the yolk-sac of a chick 

 of six somites, before the vascular area has developed. In these 

 earlier stages the glycogen is located in the yolk-sac near the 

 embryo, and it is found more or less consistently distributed 

 throughout the vascular area when this has been formed, more 

 abundant near the embryo than towards the edges. It gives to 

 the walls of the blood-vessels of this region a striking red color 

 and under high power appears as mahogany-colored masses 

 scattered among the cells. Fig. 2 shows a part of the vascular 







area of a six-day chick with the glycogenated areas represented 

 in stippling in the walls of the blood-vessels. Figs. 20, and 2b 

 show parts of the vascular areas of seventy-two-hour and four- 

 day chicks respectively, under higher magnification. In Fig. 

 2& the actual globules of glycogen are represented in black. 

 Apparently the yolk-sac furnishes a way-station in which carbo- 

 hydrates are stored as glycogen on their way from the yolk to 

 the embryonic tissue. 



This formation of glycogen in the yolk-sac makes an interesting 

 comparison with the formation of glycogen in the placenta of 

 mammals, a fact which seems well established. In mammals 

 the maternal glycogen-forming tissues are perhaps relieved in 

 in this way of the strain of a sudden call for large amounts of 

 sugar. In both cases a supply of glycogen is on hand ready- 

 formed and able to be called upon quickly. 



In the earliest chick embryos observed there appeared to be, 

 here and there, a trace of glycogen in the yolk itself in the form 

 of globules somewhere near the site of the embryo, but this fact 

 is not at present established beyond a doubt in my mind, although 

 it would accord with what Gage has observed in the eggs of the 

 Petromyzon and Amblystoma. 



In embryos of from fifteen to twenty-four hours glycogen was 

 found in the ectoderm of the head region, both in the developing 



