Embryology 287 



It is usually assumed that fertilization of the human ovum takes 

 place on about the fourteenth day of the menstrual cycle. Fertilization 

 apparently must take place in the upper part of the Fallopian tube as 

 the ovum appears to disintegrate quite rapidly. From observations on 

 other mammals, it is assumed that the ovum takes some three to four 

 days to traverse the tube and to arrive at the uterus. During this time, 

 it is undergoing cleavage. Probably when the embryo arrives in the 

 uterus it is in the blastocyst stage and the zona pellucida has disap- 

 peared. The trophoblast cells are sticky and adhere to the uterine endo- 

 metrium. About the ninth day after fertilization, the embryo has buried 

 itself in the uterine mucosa. This invasion of the uterine mucosa is 

 an active process in which the embryo may actually digest the mucosal 

 cells. 



The embryo is known as a fcfus after it acquires recognizable human 

 features. This occurs at about the beginning of the third month of de- 

 velopment. 



ORGANOGENY 



At the end of the period of early embryology, the three germ layers 

 are more or less clearly defined. It is from these germ layers that all 

 the future organs of the embryo are destined to arise. 



The position of these embryonic germ layers in the gastrula is in- 

 dicative of the organs which will arise from them. The ectoderm, being 

 on the outside, is protective and sensory; thus it gives origin to the 

 nervous system, sense organs, and the epidermis and its derivatives. 

 On the other hand, the entoderm which lines the primitive gut de- 

 velops into the lining layers of the digestive tract and the glands asso- 

 ciated with it, and later into the lining of the respiratory tract. The 

 intermediate layer, the mesoderm, is destined to give rise to the con- 

 nective and supportive tissues, the circulatory system, the excretory 

 system, the reproductive tract, muscles, and the dermis of the skin. 

 Usually the primordial germ cells are set aside before the germ layers 

 of the embryo are clearly defined; for this reason, the germinal epi- 

 thelium of the testes and ovaries cannot be said to arise from any one 

 germ layer. (Fig. 85.) 



Ectoderm. — The ectoderm is the first of the germ layers to begin 

 differentiation. At the time that the mesoderm is forming, the ecto- 

 derm flattens out and thickens as the neural plate. This plate is 

 somewhat thicker on the edges and gradually these edges fold together 



