388 - Mulficeliular Animals, Especially Man 



a profound effect upon bodily development, 

 especially in regard to the secondary sexual 

 characteristics of the female (p. 400). Before 

 the time of puberty, none of the follicles de- 

 velops, and the features peculiar to woman- 

 hood are not very clearly differentiated. 



The other endocrine function of the ovary 

 is likewise associated with the Graafian fol- 

 licles. Subsequent to ovulation, each follicle 

 undergoes a considerable change of structure. 

 The cavity of the follicle first becomes filled 

 with a small clot of blood, the blutpunkt, 

 but the blood is soon replaced by a mass of 

 yellow tissue, derived from the inner lining 

 of the follicle. This yellow body is the corpus 

 luteum, an endocrine gland that produces 

 progesterone, the so-called pregnancy hor- 

 mone (p. 401). 



The Oviducts, Uterus, and Vagina. The 

 anatomical features of the other parts of the 

 female reproductive tract are shown in Fig- 

 ure 21-9. The uterus is a pear-shaped thick- 

 walled muscular chamber, lined internally 

 with a richly vascular mucous membrane. On 

 each side of the upper margin, the uterus 

 receives a Fallopian tube, and below and 

 centrally the uterus communicates with the 

 vagina. The cervix, or lower tapered portion 

 of the uterus, protrudes somewhat into the 

 vaginal canal, which, like the uterus, has a 

 muscular wall lined internally by a well- 

 defined mucosa. Except in pregnancy, the 

 uterus is scarcely larger than a closed fist, 

 but during pregnancy the uterus enlarges to 

 accommodate the growing fetus. 



Fertilization; Implantation of the Embryo. 

 Shortly after ovulation, the egg finds its way 

 into the ostium of the Fallopian tube, which 

 bears a fringe of ciliated outgrowths, sur- 

 rounding the mouth of the oviduct (Fig. 

 21-9). Meanwhile, if copulation has occurred 

 recently, sperm will probably have succeeded 

 in leaving the vagina, traversing the uterine 

 cavity, and reaching the lumen of the Fal- 

 lopian tube. Unless fertilization occurs be- 

 fore the egg leaves the oviduct, pregnancy 

 rarely, if ever, takes place. 



Granting fertilization, the first cleavages ' 



of the egg occur in the oviduct, but soon the 

 young embryo descends into the uterus. Here 

 the embryo stimulates the uterine wall, and 

 the maternal tissues begin to surround and 

 embed the embryo. As a result of this decid- 

 ual reaction, the embryo becomes firmly im- 

 planted. After successful implantation, devel- 

 opment continues at the expense of substances 

 absorbed from the maternal blood stream. 

 At the end of about 6 weeks the embryo 

 measures almost an inch in length; and there- 

 after it may be called a fetus. The fetus 

 grows gradually, attaining a length of about 

 3 inches at 3 months; 10 inches at 5 months; 

 and 20 inches at 9 months. The gestation 

 period in man is about 10 lunar months 

 (approximately 9 calendar months), where- 

 upon the fetus is delivered through the vagi- 

 nal passage. 



NUTRITION OF THE EMBRYO 



Lower Vertebrates. The problem of sus- 

 taining the metabolism of the embryo during 

 the developmental period is relatively sim- 

 ple in the case of the lower vertebrates (for 

 example, fish and amphibians) that deposit 

 their eggs in water. Such eggs invariably 

 contain a large amount of yolk materials, 

 including protein, lipid, and carbohydrate 

 reserves, and these substances are gradually 

 mobilized and used by the cells of the em- 

 bryo as they multiply, grow, and differenti- 

 ate. Before it is used up, most of the yolk 

 comes to lie in the yolk sac, a pouchlike ex- 

 tension of the embryonic enteron (Fig. 21-11). 

 The walls of this embryonic nutritive organ 

 become highly vascularized at an early devel- 

 opmental stage, and the stored food materials 

 begin to be absorbed and distributed as soon 

 as circulation is established. The embryo is 

 surrounded by an aerated aqueous fluid, 

 from which the blood absorbs oxygen, and 

 to which it gives off carbon dioxide and other 

 waste products of metabolism. 



Land-Dwelling Vertebrates. The reptiles 

 and birds were among the earliest terrestrial 

 vertebrates, and these animals retained the 



