452 Animal Biology 



ing (ostium) of the Fallopian tube. Through the action of the cilia 

 which line the Fallopian tube the ovum, is drawn into it, and fertilization 

 usually takes place there, although, on rare occasions, the ovum may be 

 fertilized while still in the abdominal cavity. In the latter case the de- 

 veloping embryo must be removed surgically. Unless the ovum is fer- 

 tilized within a week after it is produced, it usually will not be. The 

 ovum, (egg) extrudes its first polar body at the time of ovulation and a 

 second polar body upon fertilization. The o\um is now ready to divide 

 (cleave). 



Cleavage of the fertilized ovum, (zygote) probably occurs in the Fallo- 

 pian tube. The first cleavage, extending from the animal pole to the 

 vegetal pole, results in two equal cells (blastomeres) (Fig. 224) which ad- 

 here to each other and are surrounded by an albuminous layer called the 

 zo7ia pellucida. The second cleavage is accomplished by one of the 

 blastomeres dividing longitudinally at right angles to the first cleavage, 

 to be followed by cleavage of the other blastomere, thus forming the four- 

 blastomere stage. Cleavage continues until a small sphere of blastomeres 

 the size of a pinhead is formed; this is known as the morula stage. The 

 morula is still in the Fallopian tube but probably is approaching the 

 fundus (base) of the uterus. It is thought that approximately one hun- 

 dred hours are required for the fertilized ovum to develop to the sixte en- 

 cell stage of the morula. The morula consists of (1) an outer layer of 

 cells called the trophectoderm or trophoderm (Gr. trophe, nourishment; 

 ecto, external; derm., covering) and (2) an inner cell mass. The cells of 

 the trophectoderm increase so that the outer surface is much enlarged, 

 thereby forming a saline-filled cavity (blastocoel) within the morula, 

 which is now known as the blastocyst (blastodermic vesicle) (Fig. 224). 

 Within ten days after fertilization the blastocyst has moved into the 

 uterus. The latter has developed a thick, glandular layer which is stimu- 

 lated by a ductless gland secretion, known specifically as progestin, to 

 produce a sticky fluid by means of which the blastocyst adheres to the 

 uterine wall. After a few hours the blastocyst begins to sink beneath the 

 mucous layer of the uterus (endometrium) because the latter is eroded by 

 cytolytic action of the trophectoderm cells of the blastocyst. This phe- 

 nomenon is for the purpose of ensuring nourishment for the developing 

 embryo until it can secure its own food. 



The inner cell m.ass, near the point where it contacts the trophecto- 

 derm, forms the hollow amniotic cavity (Fig. 224). The free or un- 

 attached region of the inner cell mass forms a yolk sac cavity along the 

 inner surface of the trophectoderm. The space betwen the yolk sac 



