40 FORMATION OF THE LAYERS. 



protoplasmic lines such as have been described in nuclei by 

 Flemming^, Hertwig^ and Van Benedenl 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 obser- 

 vations of Dr Kleinenberg showing that similar nuclei 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 stao^e 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 dis- 

 tinct row or even layer. If the presence of this layer is cou- 

 pled with the fact that at this period cells are beginning to 

 appear on the floor of the segmentation cavity, a strong argu- 

 ment 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 deduction 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. 



The next stage is the most important in the whole history 

 of the formation of the layers. Not only does it serve to show, 

 that the process by which the layers are formed in Elasmo- 



^ EntwickluiigsgescLiclite der Najaden, Sitz. d. k. Akad. Wien, 1875. 

 2 Morphologlsche Jahrhuch, Vol. i. Heft 3. 



^ Developpement des Mammiferes, Bid. de VAcad. de BeJgique, xl. No. 12»- 

 1875. 



