444 EMBRYOLOGY OF THE CHICK 



down and forms the first polar spindle. This rotates into position and 

 the primary oocyte is ready for its first maturation division, and later, 

 for ovulation. 



Second Period. (Ovulation, Maturation, and Fertilization.) 

 The. coordination of different functions in the body is well shown 

 by the fact that at about the time a completed egg is ready to pass into 

 the oviduct, the region of the ostium of the oviduct becomes very active 

 and actually seems to grasp the ovarian follicle which contains the pri- 

 mary oocyte. This may be due to muscular or ciliary action or it may 

 be a combination of both. The follicle then ruptures, permitting the 

 egg to be thrown out. It seems that the pressure exerted by the con- 

 traction of the fringed end of the ostium may have had something to 

 do with such rupture. The throwing of the eggs out of the follicle is 

 called ovulation. The oocyte always enters the infundibulum of the 

 oviduct with its chief axis transverse to the long axis of the oviduct, and 

 throughout its entire passage down the tube, this relation is retained. 



After the sperm has been injected into the female, they make their 

 way up the oviduct toward the ovary, seeming to gather at its end. 

 They may remain- alive and function for at least two weeks, sometimes 

 even longer. It will thus be noted that as soon as the egg has been dis- 

 charged from the follicle and has been taken into the oviduct, there are 

 millions of sperm floating about in the fluid surrounding it. A single 

 egg of the hen, unlike that in most animals, has from five to twenty-four 

 spermatozoa enter it. Such a process is known as polyspermy. 

 Polyspermy is abnormal in most animals, but it is the normal condition 

 in the hen. The egg is now fertilized. The sperm apparently affords the 

 stimulus which causes the egg to begin dividing and to form an embryo. 



The egg, after the entrance of the various spermatozoa, is not yet 

 completely mature. A process of maturation now takes place. This 

 means that the egg divides into a larger and a smaller portion, both of 

 which portions may again divide into two parts. All of the smaller por- 

 tions degenerate, one large one alone developing into a complete, fer- 

 tilized, hen's egg. The reason for these small polar bodies (as the de- 

 generating portions are called) is that one-half of the chromatin must 

 be lost in order that the new-born young may be a normal individual 

 like its parent, as explained in our studies of mitosis, maturation, and 

 genetics. 



After the second maturation division, the remaining nucleus unites 

 with a single sperm nucleus to form the first cleavage spindle, and the 

 egg is now ready to begin dividing and form a true embryo. 



Third Period. (From the Beginning of Cleavage to the time the 

 egg is laid.) 



It must be remembered here that the fertilized egg, which is to 

 become the embryo, is present in the hen's body quite a number of hours 

 before the egg is laid. In fact, from two to three days before the various 



