No. 3.] UTERUS AND EMBRYO. 431 
§ 21. General considerations. — We are nowin a position to 
compare the changes in the uterus during menstruation and 
gestation. In both cases the processes begin with tumefaction 
and hyperemia of the mucosa; they continue with hyperplasia 
of the connective tissue (the decidual cells being regarded as 
modified connective tissue corpuscles) and with hypertrophy, 
accompanied by distention and contortion of the glands ; they 
both close with casting off the superficial layers of the mucosa, 
after which follows the regeneration of the membrane. The 
essential steps, then, are the same in both cases. The difference 
is, that during the long life of the dectdua graviditatis, changes 
supervene in the tissues which do not take place during the 
rapid menstrual cycle; the mucosa of gestation is distinguished 
by the loss of both its surface and glandular epithelium, and by 
the enlargement of its connective tissue cells into so-called de- 
cidual cells. We must accordingly view the changes in the 
uterus during gestation as a prolonged and modified menstrual 
cycle. The relation in time between menstruation and the com- 
mencement of pregnancy is attributable to the menstrual pro- 
cess rendering the uterus receptive; that is to say, capable of 
receiving and retaining the ovum. We must conceive that the 
ovum has no power of initiating the development of a decidua, 
but only of modifying the menstrual process ; hence pregnancy 
can begin only at a menstrual period. The ovum, too, exercises 
this influence at a distance, for in all mammals, the earliest de- 
velopment of which is known, the ovum passes through its seg- 
mentation in the oviduct (Fallopian tube), and takes from three 
to eight days to reach the uterus; but during this period the 
change in the womb is going on. The most plausible explana- 
tion of this action of the ovum at a distance is a reflex stimulus 
passing from the oviduct to the central nervous system of the 
mother, and thence back to the uterus; the validity of this hy- 
pothesis must be tested by physiological experiment. That the 
nerves are able to effect morphological changes is already 
abundantly proven, not only by the influence of the secretory 
nerves upon gland cells, by the degeneration of muscular and 
other tissues, when their nerves are severed, but also by certain 
embryological observations tending to show that histological 
differentiation does not progress very far until the tissues are 
joined by the outgrowing nerves. 
