814 TEXTBOOK OF ZOOLOGY 



scarlike corpus albicans. In human beings the usual procedure is for 

 only one ovum to mature in one of the ovaries each four weeks 

 (approximately). At the end of the succeeding period an ovum ma- 

 tures in the other ovary. This alternation proceeds from month to 

 month in the female from puberty to menopause, except during preg- 

 nancy. When the ovum leaves the ruptured follicle of the ovary it 

 is technically in the body cavity, but in practice the funnellike 

 ostium of the oviduct receives it immediately and starts it down the 

 oviduct. It is here in the upper part of the oviduct that maturation 

 of the ovum is completed and fertilization occurs. Spermatozoa, car- 

 ried in semen, are introduced into the vagina of the female genital 

 tract in the act of copulation or coitus. These motile spermatozoa 

 swim up the oviduct and meet the ovum shortly after it enters. Fer- 

 tilization of the ovum by union with a spermatozoon occurs and the 

 zygote continues to move slowly down the oviduct. Cleavage, the next 

 step in development, takes place while the embryo proceeds along 

 the oviduct. The later stages are comi^leted normally in the uterus. 

 This process of successive cell divisoin is modified somewhat when 

 compared with that described for starfish on page 116 and frog on 

 page 533, but the same ultimate purpose of rapidly increasing the 

 cells is accomplished. Cleavage in mammals is complete and nearly 

 equal. The stages of the earlier divisions have been obtained and 

 observed in such mammals as Macacus monkey, rabbit, guinea pig, 

 rat, pig, sheep, and horse. It is thought that cleavage in the human 

 being is similar to these. One of the four cells resulting from the 

 second cleavage division is different from the others. It is the fore- 

 runner of a differentiated group of cells which soon becomes sur- 

 rounded by the other cells (Fig. 422). This enclosed group is then 

 known as the inner cells. The outer layer which surrounds these inner 

 cells is known as the trophohlast. Shortly, small pouches or vesicles 

 filled with watery fluid which is secreted by the newly formed tropho- 

 hlast, appear beneath it. These join to form a common cavity between 

 the trophoblast and the inner cells, except at one pole, where the two 

 groups of cells remain in contact. This cavity goes under the name 

 of blastocyst cavity and the whole structure, which is comparable to 

 a specialized blastula, is called a blastocyst or blastodermic vesicle. 

 The embryo's body will develop from the inner cell mass. The tropho- 

 blast becomes closely associated with the inner lining of the uterus 

 of the mother and soon plays a part in nutrition, respiration, and 

 excretion of the embryo. It sinks into the uterine lining carrying 



