REPR OD UCTION. 909 



into two equal parts, the separation taking place along the plane of nuclear 

 division (Fig. 313, D, E). Each part contains one of the new nuclei, and the 

 result of the first division is the existence of two cells, two blastomeres, in 

 place of the one fertilized ovum. The beginning of differentiation is shown 

 sometimes even as early as this, for, according to Van Beneden, in some mam- 

 mals at least, one blastomere is often somewhat larger and less granular than 

 the other. . 



Each blastomere proceeds now to divide by a similar karyokinetic process 

 into two, the result being four in all, and by subsequent divisions, eight, six- 

 teen, and more, the divisions not proceeding, however, with mathematical regu- 

 larity. By such repeated karyokinetic processes the original fertilized ovum 

 becomes a mass of small and approximately similar cells, the monda, from 

 which by continued increase of cells, morphological differentiation, and physi- 

 ological division of labor, the embryo with all its functions is destified to be 

 built up. 



Polyspermy. It happens occasionally that two or more spermatozoa enter 

 the ovum ; such a phenomenon is known as dispemiy or polyspeiiny, according 

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

 some 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 fuse together ; each centrosome divides as usual into two, making four 

 in all, which take up a quadrilateral position about the first segmentation 

 nucleus ; the chromatic figure consists of two crossed spindles ; and the egg 

 segments at once into four instead of two blastomeres. When three sperma- 

 tozoa enter, six centrosomes appear and six blastomeres result from the first 

 division, and analogous phenomena result from more complex cases of poly- 

 spermy. Apparently normal larval forms are produced from such double- 

 or multi-fertilized eggs, but as a rule their development ceases very early and 

 death occurs. 



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- 



