352 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



^1>NT •^'--^^^-S^/V. 

 ^^V'^ morula v^^r: 



aP carlv and (^ 



qostrula \^j^'^O^^y 



JOTjRNry 



cleavoqe 



H 



implon^aHon 

 beqins 



"'^s, 

 'r?'*/; 



nplontcdj/v. --IR^ ^^#1%^ ' ■'! 



implontcd 

 9 



^^n 



produces chorionic 

 qonadolrophin, ixihich 

 fur>h«r m-aintoins 

 corpus luteum 



UV 



male 



and 



female 



pronuclei 



Hijoluronidase 

 from sperms 

 breaks up corona, 

 ond sperm enters 

 eqq Second 

 maturation 

 division, beqins 



Fig. 18.14. Diagram showing essential steps in the beginning of a human indi- 

 vidual, changes in which endocrines are prominent actors in a complex scene. In 

 the ovory. An ovum and its follicular sac grow to maturity stimulated by F.S.H., 

 the follicle stimulating hormone of the anterior pituitary. The first of the two di- 

 visions of maturation (meiosis) occurs here after which the chromosome number 

 of the ovum is reduced one-half. The luteinizing hormone, L.H., of the anterior 

 pituitary stimulates the follicular sac and causes it to break and release the ovum 

 (ovulation). With luteotrophin (L.T.H.) of the anterior pituitary follicular sac to 

 transform into the corpus luteum, an important gland. In the oviduct. Entrance 

 into and movement through the oviduct are largely due to currents produced by 

 cilia. The ovum is surrounded by sperm cells. An enzyme, hyaluronidase, produced 

 by them breaks up its covering (corona) of cells. One sperm enters the ovum. This 

 is a signal for the second maturation division which is completed before male and 

 female nuclei fuse in the fertilization process. The new individual is moved through 

 the tube, at first as one cell, but soon it becomes a ball of cells. In the uterus. Under 

 the influence of the luteinizing and luteotrophic hormones, L.H. and L.T.H. of the 

 pituitary, the corpus luteum is now producing progesterone. This substance is car- 

 ried by way of the circulating blood to the uterine wall and prepares it for the 

 reception of the embryo. The white bands on the uterine wall are drawn to suggest 

 lapses of time during the processes represented. The growing embryo is surrounded 

 by coverings (chorion) that produces the hormone, chorionic gonadotrophin. This 

 is carried about by the blood, stimulates the corpus luteum which in turn produces 

 the progesterone that maintains the capacity of the uterus to hold the embryo. Mis- 

 carriages occur without this. The time in the tube varies and probably is often 

 shorter than five days. (After Dickinson. Courtesy, Ham: Histology, ed. 2. Phila- 

 delphia, J. B. Lippincott Co., 1953.) 



blood vessels are being repaired (Fig. 18.15). The physiological anticipation 

 of an embryo begins all over again. 



The human reproductive cycle is counted from the first day of menstruation. 

 All of its timing is variable, especially that of ovulation which may occur at 

 different intervals in different individuals, and even at varying intervals within 

 the same individual (Table 18.1). 



