50 SEGMENTATION AND FORMATION OP THE LAYERS. 



have examined other segmenting ova to see if the fact was one 

 of general application, with negative results. 



The cells of segmenting ova are generally so closely applied 

 together and the protoplasmic strands so hidden by food-yolk, 

 that it is difficult to be certain of the point either way. But 

 with ova in which the segments are slightly separated from one 

 another — and I believe there are such though I have never seen 

 them — the observation ought to present no special diflSculty. 



Indeed it is a well-known fact that an incomplete separation 

 of the cells is found in the early stages of the segmentation of 

 centrolecithal eggs ; but it has always been assumed that this 

 was a temporary phase, and that the segments eventually sepa- 

 rated. We now know, thanks to the researches of Heathcote 

 (4), that this separation does not occur in the centrolecithal egg 

 of the Myriapod, Julus; and it seems to me extremely probable 

 that his results for this form will be found on careful examina- 

 tion applicable to other similar ova. 



BlBLIOGKAPHY. 



1. Flemming, W. — ' Zellsubstanz, Kern und Zelltheilung,' Leipzig, 1882. 



2. Feaipont, J. — 'Archives de Biologie," vol. v. 



3. Gbtjber, a. — "Kerne u. Kerntheile bei den Protozoan," 'Zeit. f, w. 



Zool.,' Bd. xl. 



4. Heathcote, F. G. — " The Development of Julus," ' Proc. Royal Soc, 



1886. 



5. Heitzmann. J. — " Untersuch. iib. d. Protoplasma," and " Das Verhalt- 



niss zwischen Protoplasma u. Grundsubstanz im Thierkorper," ' Sitz- 

 ungber d. Kais. Akad. Wiss.,' Wien, Bd. Ixvii, 1873. 



6. HuTTON, Captain F. W. — "On Peripatus novse-zealandise," 



•Ann. and Mag. Nat. Hist.,' 4th ser., vol. xviii, 1876. 



7. JiCKELi, C. F. — " Ueber die Kernverhaltnisse der Infusorien," ' Zool. 



Anzeiger,' 1884. 



8. Kennel, J. — " Entwickelungsgeschichte von Peripatus Edwardsii," 



' Arbeiten u. d. z. d. d. Wurzburg,' Bd. vii. 



9. Klein, E. — " Observations on Structure of Cells and Nuclei," ' Quart. 



Journ. Micr. Sci.,' vol. xviii. 

 10. Klein, E. — "Observations on Structure of Cells and Nuclei," 'Quart. 

 Journ. JMicr. Sci.,' vol. six. 



