REPRODUCTION. All 



Polyspermy. — It happens occasionally that two or more spermatozoa enter 

 the ovum ; such a phenomenon is known as dispermy or 'polyspermy, according 

 to the number of entering sperms. Each sperm with its nucleus and centro- 

 somc becomes a male pronucleus and proceeds to conjugate with the female 

 pronucleus. In the case of dispermy the one female and the two male pro- 

 nuclei i'u^c together; each centrosome gives place as usual to two, making 

 four in all, which take up a quadrilateral position about the firsl segmenta- 

 tion-nucleus; the chromatic figure consists of two crossed spindles; and the 

 egg segments at once into four instead of two blastomeres. Analogous phe- 

 nomena result from more complex cases of polyspermy. In such double- or 

 multi -fertilized eggs development may proceed to some distance, but typical 

 larval forms are not produced, and death occurs early. 



During cleavage the ovum proceeds, after the manner of the non-fertilized 

 ovum, slowly along the Fallopian tube and enters the uterus. Unlike the non- 

 fertilized ovum, however, the morula is not cast out of the body, but remains 

 and undergoes further development. The morphological development of the 

 embryo in utero does not fall within the scope of the present article. Some 

 attention may, however, be given to the immediate environment of the develop- 

 ing child and its relations to the maternal organism. 



Decidua Graviditatis. — While the segmentation of the ovum is proceed- 

 ing within the Fallopian tube, the uterus prepares for the future guest by begin- 

 ning to undergo a profound change, probably being stimulated to activity re- 

 flexly by centripetal impulses originating in the walls of the tube through con- 

 tact with the ovum. This change comprises an enlargement of the whole uterus 

 and a great and rapid growth in thickness of its mucosa and its muscular 

 coat. At first the alterations are not unlike the phenomena of growth pre- 

 ceding the menstrual flow, but, as they proceed, they become much more pro- 

 found than those. The supply of blood to the walls is greatly increased, the 

 vessels forming large irregular sinuses within the mucosa. The supply of lymph 

 is increased. The glands become tortuous and dilated into flattened cavernous 

 spaces, and their walls atrophy, the epithelium breaking down except in their 

 deepest parts. The mucosa is thus converted into a spongy tissue, the frame- 

 work of which contains numerous large irregular cells, derived probably from 

 the original connective tissue and called decidual cells. The musculature is 

 greatly thickened by an increase, partly in number and partly in size, of its 

 constituent fibres, and the nerve-supply is Increased. These general structural 

 changes proceed through the early part of gestation and arc accompanied In- 

 special changes to be discussed later. It is not definitely known how far the 

 alterations have gone before the advent of the segmented ovum into the uterus. 

 With the latter instead of the unimpregnated ovum present in the Fallopian 

 tube, the hypertrophied uterine mucosa docs not break away as in menstrua- 

 tion, but remains, and henceforth is called the decidua graviditatis, special 

 names being given to special parts. Entering the uterus, the ovum attaches 

 itself in an unknown manner to the wall of the womb. The part of the mucous 



