CHAPTER VIII 



CELL-DIVISION AND DEVELOP.MENT 



" Wir konnen deninach endlich den Satz aufstellen, dass sammtliche im entwickelten 

 Zustande vorhandenen Zellen oder Aequivalente von Zellen durch eine fortschreitende 

 Gliederung der Eizelle in morphologisch ahnliche Elemente entstehen, und dass die in einer 

 embryonischen Organ-Anlage enthaltenden Zellen, so gering auch ihre Zahl sein mag, 

 dennoch die ausschliessliche ungegliederte Anlage fiir sammtliche P^ormbestandtheile der 

 spateren Organe enthalten." Remak.^ 



Since the early work of Kolliker and Remak it has been recog- 

 nized that the cleavage or segmentation of the ovum, with which 

 the development of all higher animals begins, is nothing other than 

 a rapid series of mitotic cell-divisions by which the egg splits up 

 into the elements of the tissues. This process is merely a contin- 

 uation of that by which the germ-cell arose in the parental body. 

 A long pause, however, intervenes during the latter period of its 

 ovarian life, during which no divisions take place. Throughout this 

 period the egg leads, on the whole, a somewhat passive existence, 

 devoting itself especially to the storage of potential energy to be used 

 during the intense activity that is to come. Its power of division 

 remains dormant until the period of full maturity approaches. The 

 entrance of the spermatozoon arouses in the egg a new phase of 

 activity. Its power of division, which may have lain dormant for 

 months or years, is suddenly raised to the highest pitch of intensity, 

 and in a very short time it gives rise by division to a myriad of de- 

 scendants which are ultimately differentiated into the elements of 

 the tissues. 



The divisions of the egg during cleavage are exactly comparable 

 with those of tissue-cells, and all of the essential phenomena of 

 mitosis are of the same general character in both. But for two 

 reasons the cleavage of the egg possesses a higher interest than 

 any other case of cell-division. First, the egg-cell gives rise by divi- 

 sion not only to cells like itself, as is the case with most tissue-cells, 

 but also to many other kinds of cells. The operation of cleavage is 

 therefore immediately connected with the process of differentiation, 

 which is the most fundamental phenomenon in development. Second, 

 definite relations may often be traced between the planes of division 

 and the structural axes of the adult body, and these relations are 



^ Untei'suchnngen. 1S55, p. 140. 

 361 



