No. 2.] FORMATION OF THE FISH EMBRYO. 463 



germ-ring, and, to a less degree, the cells of the extra-embryonic 

 region are found also to divide. 



Whether these divisions occur sufficiently often in the extra-embry- 

 onic region to account for the larger number of cells of that region 

 in later stages must remain an open question. From the calculations 

 given in the preceding pages we see that if each nucleus between 

 the stages Fig. 2, A, and Fig. 6, A, had divided three (and a half) 

 times the full number of cells would result. All things considered, 

 I am inclined to believe that two processes take place in the inner 

 layer of the extra-embryonic region, by means of which new cells are 

 added to that region. First by cell-division more cells are produced, 

 and secondly additions are also made from the germ-ring while the 

 latter grows over the yolk-sphere. 



The observations here recorded make it probable that nearly all 

 of the material of the germ-ring, however, finds its way, ultimately, 

 into the embryo. At first probably a larger amount passes in and 

 later a smaller amount, but at neither period is there sufficient mate- 

 rial in the ring to form the sides of the embryo. 



Perhaps it may be best to restate here the factors that seem to me 

 to be at work during the formation of the teleostean embryo : First 

 there is an early accumulation of material that reaches from the cen- 

 tre of the blastoderm to the germ-ring on one side. Part of this 

 material has never been at the edge of the disc. The material lying at 

 the edge of the blastoderm continues out into the germ-ring on each 

 side. 



Secondly, in the so-called caudal swelling, at the posterior end of 

 the embryo, there is at each stage a large amount of material. We 

 find that much of this material is continually transferred backwards, 

 because volumetric comparisons show that it is not all used up to 

 form the regions of the embryo that lie at corresponding distances 

 from the head. To this caudal swelling (or just in front of it) new 

 material is always being added. This is the material flowing into 

 the embryo from the germ-ring. When, finally, the blastopore closes, 

 the caudal swelling has exhausted all of its material in the formation 

 of the embryo, and, at the same time, the material of the germ-ring 

 has passed in. Thirdly, in addition to the preceding factors that go 

 to form the embryo, the measurements of the later stages seem to 

 show that there is an actual elongation of the embryo as a whole 

 during the later periods of overgrowth. This factor does not seem 

 to be so important as the preceding two. It is possible that the 

 material of the embryo only becomes more concejitrated during the 



