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Nr. 11. 1878. 

 Sarasin, C. P. Eeifung u. Furohurig des Eeptilieneies. Arbeiten a. d. 



zool.-zoot. Inst. Wurzburg. Bd. VI. p. 159. ] 883. 

 Schneider. Untersuchungen fiber Plathelminthen. Jahrb. d. oberhessischen 



Gesellsoh. f. Natur- u. Heilkunde. 1873. 

 Strasburger. Zellbildung und Zelltheilnng. 3. Aufl. Je?ia 1875. 



CHAPTER IV. 



GENHBAL DISCUSSION OF THE PRINCIPLES OF DEVELOP- 

 MENT. 



A SIMPLE principle has exclusively controlled the embryonic pro- 

 cesses Htherto considered. By means of the cleavage of the egg- 

 substance, or cell-division, alone the originally simple elementary 

 organism has been converted into a cell-colony. This presents the 

 simplest conceivable form, inasmuch as it is a hollow sphere, the 

 wall of which is composed of one or several layers of epithelial cells. 

 But the principle of cell-division is not adequate for the production, 

 out of this simple organism, of more complicated forms with dissimilar 

 organs, such as the adult animals are ; further progress in develop- 

 ment can be brought about from this time forward only by the 

 supervention of two other principles, which are likewise simple ; 

 namely, the principle of unequal growth in a cell-membrane, and 

 the principle of the division of labour, together with the histological 

 differentiation connected with it. 



Let us consider first the principle of unequal growth. When in a 

 cell-membrane the individual elements continue to divide uniformly, 

 the result will be either a thickening or an increase in the surface of 

 the membrane. The former takes place when the plane of division 

 has the same direction as the surface of the membrane, the latter 

 when it is perpendicular to the surface. With the increase in the 

 extent of surface the cells which were at first present are uniformly 

 and gradually crowded apart by the introduction of the new daughter- 

 cells, inasmuch as they are soft and plastic, and are joined together 

 only by means of a soft cementing substance. Were we to assume 

 that only such a growth took place in the case of the blastula during 

 its further development, nothing else could come of it except an ever 

 larger and thicker- walled hollow sphere of cells. 



