DEVELOPMENT OF THE EMBRYO 



445 



layers of white, yolk, and shell, have encircled it. The heat from the 

 mother's body has caused the embryo to begin to form, so that by the 

 time the egg is laid, the embryo is already several days old. It is there- 

 fore essential that the student understands in detail, exactly what has 

 already happened in the mother's body before the egg passes to the outer 

 world. 



The first cleavage furrow can be seen about three hours after the 

 ovum has been discharged from the follicle. During this period the egg- 

 has passed along the entire glandular portion of the oviduct. The glands 

 themselves have secreted the most dense portions of albumen and also 

 the chalazae, it being remembered that the yolk was already laid down 

 before ovulation. The egg is carried along principally by peristaltic 

 action of the walls of the oviduct. Then, as the egg itself rotates, the 

 germ disc comes to describe a spiral path, which explains the spiral ar- 

 rangement of the albumen around the yolk. The egg then traverses the 

 isthmus for approximately an hour where the shell membrane is secreted 

 over the dense albumen. The fluid layer of albumen is secreted both in 

 the isthmus and the upper part of the uterus. . The fluid layer of the 

 albumen passes through the shell membrane which has already been laid 

 down, and it takes from five to seven hours after the egg enters the 

 uterus before this is completed. But, before this takes place, the shell 

 substance itself has already begun to be laid down on the shell mem- 

 brane. Usually twelve to sixteen hours are necessary to complete the 

 passage through the uterus and vagina. At the end of this time twenty- 

 one to twenty-seven hours have already elapsed since ovulation took 

 place. Gastrulation has begun, and the egg is laid. 



We have already mentioned that if the egg reaches the vagina, ready 

 to be laid, during the main portion of the day, it will be laid that day, 

 but if it should be ready for laying after four or five o'clock in the after- 

 noon, it will be retained in the vagina until the following day, thus caus- 

 ing some embryos in freshly-laid eggs to be approximately twelve hours 

 older than others. It is for this reason that there is always considerable 

 variation, even when eggs have been incubated for the same number of 

 davs. 



TABLE (After Kellicott) SHOWING THE CHIEF EVENTS 

 THE EARLY HISTORY OF THE HEN'S EGG 



IN 



Hours after 



Ovulation 







to 3 



Location in 



Oviduct 

 Infundibulum 



Glandular 

 Portion 



Action of Oviduct 

 Reception of Ovum 



Secretion of chala- 

 zae, chalaziferous 

 and dense albumen 

 layers. 



Action of Germ Disc 



Maturation and Fer- 

 tilization. 



First cleavage fur- 

 row. 



