

SEGMENTATION OF THE FERTILIZED OVUM 



45 



tozoon is about to pierce, the yolk is drawn out into a conical elevation, termed 

 the cone of attraction. As soon as the spermatozoon has entered the yolk, the per- 

 ipheral portion of the latter is transformed into a membrane, the vitelk'ne membrane 

 which prevents the passage of additional spermatozoa. Occasionally a second 

 spermatozoon may enter the yolk, thus giving rise to a condition of polyspermy: 

 when this occurs the ovum usually develops in an abnormal manner and gives rise 

 to a monstrosity. Having pierced the yolk, the spermatozoon loses its tail, while 

 its head and connecting piece assume the form of a nucleus containing a cluster of 

 chromosomes. This constitutes the male pronucleus, and associated with it there are 

 a centriole and centrosome. The male pronucleus passes more deeply into the yolk, 

 and coincidently with this the granules of the cytoplasm surrounding it become 

 radially arranged. The male and female pronuclei migrate toward each other, and. 

 meeting near the center of the yolk, fuse to form a new nucleus, the segmentation 

 nucleus, which therefore contains both male and female nuclear substance; the 

 former transmits the individualities of the male ancestors, the latter those of the 

 female ancestors, to the future embryo. By the union of the male and female 

 pronuclei the number of chromosomes is restored to that which is present in the 

 nuclei of the somatic cells. 



ot. 



FIG. 9. First stages of segmentation of a mammalian ovum. Semidiagrammatic. (From a drawing by Allen 

 Thomson.) z.p. Zona striata. p.gl. Polar bodies, a. Two-cell stage, b. Four-cell stage, c. Eight-cell stage 

 a. e. Morula stage. 



SEGMENTATION OF THE FERTILIZED OVUM. 



The early segmentation of the human ovum has not yet been observed, but 

 judging from what is known to occur in other mammals it may be regarded as 

 certain that the process starts immediately after the ovum has been fertilized, 

 i. e., while the ovum is in the uterine tube. The segmentation nucleus exhibits 

 the usual mitotic changes, and these are succeeded by a division of the ovum into 

 two cells of nearly equal size. 1 The process is repeated again and again, so that 



1 In the mammalian ova the nutritive yolk or deutoplasm is small in amount and uniformly distributed through- 

 out the cytoplasm; such ova undergo complete division during the process of segmentation, and are therefore termed 

 ashc. In the ova of birds, reptiles, and fishes where the nutritive yolk forms by far the larger portion of the 

 sgg, the cleavage is limited to the formative yolk, and is therefore only partial; such ova are termed meroblastic. Again, 

 it has been observed, in some of the lower animals, that the pronuclei do not fuse but merely lie in apposition. At 

 s commencement of the segmentation process the chromosomes of the two pronuclei group themselves around the 

 itor of the nuclear spindle and then divide ; an equal number of male and female chromosomes travel to the opposite 



f the spindle, and thus the male and female pronuclei contribute equal shares of chromatin to the nu< " ' 

 tne two cells which result from the subdivision of the fertilized ovum 



clei of 



