46 PROFESSOR VAN BENEDEN. 



divide in their turn. I have ascertained that the time which 

 elapses between two successive phases of cleavage is shorter 

 and shorter as the cells diminish in volume and in consequence 

 of such diminution. I pointed out the same fact after having 

 studied the segmentation of the egg of Gammarus locmta, and it 

 is remarkably evident in the Rabbit. I will not delay over a 

 detailed description of the segmentation. This phenomenon 

 has been often described and figured, and, moreover, I was not 

 able to study either the order or mode of appearance of the 

 successive grooves with sufficient completeness to be able to add 

 anything to what is already known. All my attention was con- 

 centrated on the intermediate layer, and I endeavoured to see as 

 exactly as possible the modifications which it underwent during 

 the earliest period of embryonic development. I have represented 

 in the Plate two phases of the segmentation properly so called. 

 Figure 2 shows the stage at which the development had arrived 

 about eleven o'clock. At this moment I could not discover any 

 trace of nuclei in the cells of the germ whilst still living; but 

 on treating the eggs with osmic acid, then by weak alcohol, 

 colouring them subsequently by means of picro-carmine, I was 

 able to demonstrate the existence in each of the segmentation 

 spheres of a tine spherical nucleus, homogeneous and devoid of 

 nucleoli. When the eggs are allowed to die on the object-slide 

 the protoplasm of the spheres becomes cloudy and finely granular, 

 taking at the same time a slightly brownish tint. There appears 

 then at the centre of each sphere a large clear spot, ill-detined 

 and homogeneous in appearance. These spots are simply the 

 nuclei. Acetic acid of 1 per cent, also renders them very 

 obvious. At this })hase of development a very sharp line sepa- 

 rates the segmented germ from the " intermediate layer." The 

 latter has retained precisely the same contour as that seen in 

 the phase previously described. It shows clearly its median 

 lens and its peripheral welt. I could not discover in any part of 

 this layer the least trace of a nucleus. Neither during life nor 

 after the death of the ^^^j^^, when the intermediate layer shrinks 

 somewhat and when nuclei make their aj)pearancein the segmen- 

 tation spheres, nor indeed by means of osmic acid, nor by picro- 

 carmine, nor by acetic acid, nor by hematoxylin, could 1 succeed 

 in causing nuclei to appear in this layer. I think myself, then, 

 entitled to affirm that at this phase of the development of 

 the egg there exists no trace of nucleus in the intermediate 

 layer. 



At five o'clock in the afternoon the eggs had arrived at the 

 stage which 1 have represented in fig. '3. The cleavage-disc 

 presents, taken as a whole, the form of a plano-convex lens ; it 



