92 REGULAR SEGMENTATION. 



Nor is it in the least proved by the fact (shewn by Hallez) that 

 the plane of division of the cell always bears a definite relation 

 to the direction of the axis of the nucleus. 



The arguments by which Kleinenberg (93) attempted to de- 

 monstrate that cell division was a phenomenon caused by altera- 

 tions in the molecular cohesion of the protoplasm of the ovum 

 still in my opinion hold good, but recent discoveries as to the 

 changes which take place in the nucleus during division probably 

 indicate that the molecular changes which take place in the co- 

 hesion of the protoplasm are closely related to, and possibly 

 caused by, those in the nucleus. These alterations of cohesion 

 are produced by a series of molecular changes, the external indi- 

 cations of which are to be found in the visible alterations in the 

 constitution of the body of the cell and of the nucleus prior to 

 division. 



BIBLIOGRAPHY. 



In addition to the papers cited in the last Chapter, vide 



(101) W. F lemming. " Beitrage z. Kenntniss d. Zelle u. ihrer Lebenserschei- 

 nungen." Archiv f. mikr. Anat., Vol. xvi., 1878. 



(102) E. Klein. "Observations on the glandular epithelium and division of 

 nuclei in the skin of the Newt." Quart. J. of Micr. Science, Vol. xix., 1879. 



(103) Peremeschko. " Ueber d. Theilung d. thierischen Zellen." Archiv f. 

 mikr. Anat., Vol. xvi., 1878. 



(104) E. Strasburger. " Ueber ein z. Demonstration geeignetes Zelltheilungs- 

 Object." Site. d. Jenaischen Gesell.f. Med. u. Naturwiss., July 18, 1879. 



External Features of Segmentation. 

 In the simplest known type of segmentation the ovum first 



,- 



FIG. 38. VARIOUS STAGES IN PROCESS OF SEGMENTATION. (After Gegenbaur.) 



of all divides into two, then four, eight, sixteen, thirty-two, sixty- 

 four, etc. cells (fig. 38). These cells so long as they are fairlylarge 

 are usually known as segments or spheres. At the close of such 



