10 EMBEYOLOGY OF THE LOWEE VERTEBRATES ch. 



indicated meridional in position but become displaced somewhat so 

 as to be in the case of the micromeres perpendicular to the first 

 (sagittal) division-plane or in that of the macromeres slightly oblique. 



Fifth division. Each blastomere divides again, 1 the smaller 

 blastomeres towards the apical pole dividing rather earlier than the 

 others, and the result is that there are now thirty-two blastomeres in 

 all, arranged in eight meridional rows of four cells each, the cell at 

 the lower (abapical) end being decidedly larger than the others. 

 Between these four large elements a wide opening is present 

 (Fig. 3, G) leading into a space which made its appearance as a 

 little chink between the blastomeres of the four-cell stage but which 

 has since then increased greatly in size. This space in the interior 

 of the egg is the blastocoele or segmentation cavity. 



From this period onwards the segmentation process becomes less 

 regular. There has already shown itself a tendency for the larger 

 blastomeres towards the lower pole to lag behind somewhat. And 

 the arrangement of the blastomeres becomes less regular as they 

 become smaller and fit more closely together. In particular the 

 bilateral symmetry in the arrangement of the blastomeres which is 

 conspicuous in most of the eggs during the earlier stages (Cerfontaine) 

 ceases to be apparent. 



To summarize the later phases of segmentation it may simply 

 be said that the blastomeres go on dividing, the segmentation cavity 

 increases in size, its communication with the exterior closes up, and 

 there is formed eventually a blastula of approximately spherical shape. 

 The wall of the blastula is composed of a single layer of cells those 

 towards the apical pole being smaller and less rich in yolk than 

 those on the opposite side (Fig. 3, I). 



RANA 



In the case of the Frog we have an egg in which as compared 

 with that of Amphioxus there is present a much greater proportional 

 amount of yolk and which in consequence serves well to illustrate 

 the nature of the influence of yolk upon segmentation. 



The process of segmentation begins with the appearance, in the 

 region of the apical pole, of a small dimple on the surface of the egg 

 which gradually lengthens out to form the first meridional furrow 

 (a). The furrow gradually extends downwards over the surface 

 of the egg (Fig. 5, A) and becomes completed by reaching the 

 lower pole after about an hour and a quarter. 2 It also extends 

 inwards from the surface and finally bisects the egg into two 

 hemispheres. 



The second furrow (/?) is also meridional and is in a plane 



1 As there are marked discrepancies between the accounts given by different 

 observers we may take it as probable that there is considerable variability in the 

 details of segmentation about this stage. 



2 See, however, later for caution in reference to the time factor in development. 



