334 F. M. BALFOUR. 



most marked at the point where the embryo will shortly 

 appear. I will call it the " embryonic rim." • 



These points are still better seen from sections than from 

 surface views, and will be gathered at once from an inspection 

 of PI. XIII, fig. 4. 



The epiblast has become still more columnar, and is 

 markedly thicker in the region where the embryo will ap- 

 pear. But its most remarkable feature is that at the outer 

 edge of the " embryonic rim " (e ?•) it turns round and becomes 

 continuous with the lower layer cells. This feature is most 

 important, and involves some peculiar modifications in the 

 development. I will, however, reserve a discussion of its 

 meaning till the next stage. 



The only other important feature of this stage is the appear- 

 ance of a layer of cells on the floor of the segmentation 

 cavity. 



Does this layer come from an ingrowth from the thickened 

 edge of the blastoderm, or does it arise from the formation of 

 new cells in the yolk ? 



It is almost impossible to answer this question with cer- 

 tainty. The following facts, however, make me believe that 

 the newly formed cells do play an important part in the 

 the formation of this layer. 



(1) The presence at an earlier date of almost a row of 

 nuclei under the floor of the segmentation cavity (PI. XIII, 

 fig. I). 



(2) The presence on the floor of the cavity of such large 

 cells as those represented in fig. 1, b d, cells which are very 

 different, as far as the size and gi'anules are concerned, from 

 the remainder of the cells of the blastoderm. 



On the other hand, from this as well as other sections, 

 I have satisfied myself that there is a distinct ingrowth of 

 cells from the embryonic swelling. It is therefore most 

 probable that both these processes, viz. a fresh formation and 

 an ingrowth, have a share in the formation of the layer of 

 cells on the floor of the segmentation cavity. 



In the next stage we find the embryo rising up as a 

 distinct body from the blastoderm, and I shall in future 

 speak of the body, which now becomes distinct as the em- 

 bryo. It corresponds with what Kupffer (loc. cit.) in his 

 paper on the " Osseous Fishes " has called the " embryonic 

 keel." This starting-point for speaking of the embryo as 

 a distinct body is purely arbitrary and one merely of con- 

 venience. If I wished to fix more correctly upon a period 

 which could be spoken of as marking the commencing forma- 

 tion of the embryo, I should select the time when structures 



