. 



FORMATION OF THE LAYERS. 253 



It has not been possible for me to satisfy myself as to the 

 exact meaning of the lines dividing these nuclei into a number 

 of distinct areas. My observations leave the question open as to 

 whether they are to be looked upon as lines of division, or as 

 protoplasmic lines such as have been described in nuclei by 

 Flemming 1 , Hertwig 2 and Van Beneden 3 . The latter view ap- 

 pears to me to be the more probable one. 



Such are the chief structural features presented by these 

 nuclei, which are present during the whole of the earlier periods 

 of development and retain throughout the same appearance. 

 There can be little doubt that their knobbed condition implies 

 that they are undergoing a rapid division. The arguments 

 for this view I have already insisted on, and, in spite of the 

 observations of Dr Kleinenberg shewing that similar nu- 

 clei of Nephelis do not undergo division, the case for their 

 doing so in the Elasmobranch eggs is to my mind a very 

 strong one. 



During this stage the distribution of these nuclei in the yolk 

 becomes somewhat altered from that in the earlier stages. 

 Although the nuclei are still scattered generally throughout the 

 finer yolk- matter around the blastoderm, yet they are especially 

 aggregated at one or two points. In the first place a special 

 collection of them may be noticed immediately below the floor 

 of the segmentation cavity. They here form a distinct row 

 or even layer. If the presence of this layer is coupled with the 

 fact that at this period cells are beginning to appear on the floor 

 of the segmentation cavity, a strong argument is obtained for 

 the supposition that around these nuclei cells are being produced, 

 which pass into the blastoderm to form the floor. Of the actual 

 formation of cells at this period I have not been able to obtain 

 any satisfactory example, so that it remains a matter of de- 

 duction rather than of direct observation. 



Another special aggregation of nuclei is generally present 

 at the periphery of the blastoderm, and the same amount of 

 doubt hangs over the fate of these as over that of the previously 

 mentioned nuclei. 



1 "Entwicklungsgeschichte der Najaden," Sits. d. k. Akad. IVien, 1875. 



2 Morphologische Jahrbuch, Vol. I. Heft 3. 



'Developpement des Mammiferes," Bui. de r Acad. de Bdgiqiic, XL. No. 12, 1875. 



a 



