SEGMENTATION OF THE OVUM 



While the present work deals primarily with the adult human organism in the 

 stage of maturity, reference is made also to its changes according to age. Although 

 these changes for the various systems of organs are described under the ap- 

 propriate sections, it is desirable to consider first -some of the more fundamental 

 features pertaining to the body as a whole. This applies particularly to the 

 earlier embryonic period, which includes the more general phases of morpho- 

 genesis. No attempt will be made to describe fully the process of development, 

 the details of which are to be found in text-books of embryology. 



Segmentation of the ovum.^The human body, like all living organisms, arises 

 from a single cell, the egg-cell or ovum. An early stage in the development of the 

 ovum is shown in fig. 4, and a later stage, approaching maturity, in fig. 5. The 

 mature human ovum is about 0.2 mm. in diameter. In the uterine C Fallopian) 

 tube, the fertilised ovum undergoes segmentation, the various stages of which are 

 represented in figs. 6 and 7. 



Fig. 7. — Diagram of Sectiox 

 through a mammalian ovum at the 

 Morula Stage. 





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Fig. 8. — Diagram of Section of 

 A Mammalian Ovum Showing the 

 Inner Cell Mass. 





INNER C^LL MASS 





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I 



While the processes of maturation, fertihsation and segmentation have not as yet been ob- 

 served in the human ovum, the evidence of comparative anatomy makes it very probable that in 

 all essential respects these processes are like those found in other mammals. As a result of the 

 successive divisions of the ovum in segmentation, a spherical mass of cells, the morula (fig. 7) is 

 formed. In this mass, an excentric cavity forms (fig. 8) whereby the mass is transformed into a 

 hollow vesicle. The wall of this vesicle is probably formed throughout the greater part of its 

 extent by a single laj-er of cells; but at one point of the circumference there is a group of cells 

 termed the inner cell mass (fig. 8) . Probably about this time the ovum enters the uterine cavity, 

 and through the activity of the outer layer of cells {trophoblast) becomes embedded in the uterine 

 mucosa. 



Formation of the embryonic disc and germ layers.— In the earliest human 

 embryos which have been described, development has already proceeded beyond 



Fig. 9. — Diagram Showing the Relations of the Germ Layers in an Early Embryo. 

 Ac, amniotic cavity, Uned by ectoderm. D, yolk-sac, lined by endoderm (En). Me,Me , 

 mesoderm, C, extra-embryonic calom. B, chorion. T, trophoblast. (McMurrich.) 



the stage represented by fig. 8, and has reached that of fig. 9. Within the inner 

 cell mass, two cavities have appeared. The more superficial fac) is the amniotic 



