DEVELOPMENT 



195 



70c. Organogenesis. When the embryo has assumed the 

 phase that is represented by the gastrula the process of formation 

 of the organs of the adult phase begins. It must be remembered, 

 however, that the gastrula itself is an organism and it may be 

 (and very often is) a viable organism, mobile, irritable, and capable 

 of independent existence in the wild state and carrying out all 

 functional activities except those of reproduction. In the cases 

 of embryos that undergo continuous development the gastrula (or 

 the phase roughly corresponding to this) is, of course, dependent, 

 for continued existence as a living thing, on the maternal nutritive 

 substances, or on those that are included in the egg. 



t 





1 



2 \ 



OGOecDOG 



Fig. 28.- 



6 7 



-Diagrams of the Cell-divisions ix Developing Organs. 



The formation of the organ-anlagen proceeds by division of 

 the germ-layer cells (we still refer to quite typical embryogenies, 

 such as those of the chick). The process is that of cell-division, 

 the division-planes having strict tendency. Complicated as the 

 process may appear to be in such a description as that which 

 follows it is apparently exceedingly simple and " natural " when 

 one actually studies it in chick, or frog embryos that are observed 

 from day to day throughout the period of organogeny. 



Consider an epithehum, that is a sheet of cells, one layer thick. 

 Let the division-planes be all parallel to each other and per- 

 pendicular to the surface of the epithelium (Fig. 28, 7), and 

 obviously the sheet of epithelium will grow lengthwise, still 

 remaining a sheet of one cell in thickness. Let the division-planes 

 be all parallel to each other and parallel to the surface of the 



