22 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



condition is reached, the cells of the basal layer being still connected 

 by a thick continuous stratum of protoplasm. By this time it is 

 found that the nuclear divisions of the basal layer are clearly lagging 

 behind those of the other layers. As. segmentation proceeds further 

 the continuous basal sheet of protoplasm decreases relatively in 

 thickness. For a time (Fig. 13, E) bulgings of its upper surface 

 indicate that it is giving off cells into the overlying layer but as 

 the thinning process goes on these become less and less numerous. 



H. Virchow distinguished three zones -in the basal sheet of 

 protoplasm — marginal, intermediate and central, although the latter 



Fig. '13. — Vertical sections through the blastoderm of a Teleost (Salmo fario) illustrating 

 the process of segmentation. (After Kopsch, 1911.) 



A, end of second division. Section perpendicular to .plane of first furrow which is therefore seen 

 cut across. B, commencement of fourth division. Plane of section as in A. The division surfaces 

 of the third division are seen to curve inwards so as to meet the first division surface. As a result 

 the latter has become distorted and no longer forms a plane. C, middle of sixth division. D, be- 

 ginning of eighth division. E, beginning of tenth division. F, 62-hour blastoderm. (The dark 

 portion at the top of Fig. B represents the free surface bounding the second furrow : the dark tone at 

 the lower edge of each figure represents yolk.) 



is not quite central but situated rather towards the posterior or 

 embryonic edge of the blastoderm. The intermediate zone is marked 

 off from the others' by the fact that the thinning process has there 

 progressed farther. 



Up to about the twelfth division the nuclei all through the 

 blastoderm divide practically synchronously except those of the 

 basal layer which as already indicated lag behind. Soon after this 

 however (from about the 41st hour — Kopsch) the divisions become 

 irregular. 



The basal layer becomes the yolk -syncytium : the cell limits 

 visible on its upper side become obliterated and it becomes more and 

 more flattened out. Although its nuclei undergo repeated mitosis 



