SOME ASPECTS OF FORM AND GROWTH 



THE changes in the form of animals during growth and 

 development are included under the general term mor- 

 phogenesis. As an introduction to the discussion of certain phases 

 of morphogenesis in the vertebrate organism, the typical stages 

 in human development will be 

 reviewed briefly (Figure 60). 

 These will serve as a basis for 

 comparison with other ani- 

 mals. As in most living organ- 

 isms, we begin with a single 

 cell (A), the egg-cell or 

 ovum. The human ovum is 

 about one-tenth of a milli- 

 meter in diameter, or large 

 enough to be barely visible to 

 the naked eye. The fertilized 

 ovum divides repeatedly to 

 form a spherical mass of cells, 

 the morula (B). (This and 

 the next stage have not yet 

 actually been found in man, 

 but are inferred from their 

 occurrence in related forms.) 

 Fluid then accumulates within 

 the morula, forming a some- 

 what distended, hollow blas- 

 tula (C). On the upper side of the blastula, a rounded group of 

 cells, the inner cell mass (km), projects into the cavity. 



Within the inner cell mass, two secondary cavities soon ap- 

 pear (D). The upper becomes the amniotic cavity (am), the 

 lower indicates the yolk sac (ys). Between these two cavities 



Figure 60. Early stages of morphogenesis. 



A. Human ovum or egg-cell. 



Morula with many cells, resulting from 

 division of the ovum. A segmentation 

 cavity (sc) is beginning to form. 

 The segmentation cavity (sc) has ex- 

 panded, forming a hollow sphere (blas- 

 tula), with most of the cells clumped 

 above to form the inner cell mass (icm). 

 Two cavities have appeared in the in- 

 ner cell mass: an upper (am), the am- 

 niotic cavity; and a lower (ys), the 

 cavity of the yolk sac. Between these 

 two cavities lies the embryonic disk, with 

 the three germ layers. 



B. 



D 



