No. 2.] FORMATION OF THE FISH EMBRYO. 437 



I have studied in serial sections many of the embryos that 

 have developed from those eggs where one blastomere had been 

 removed. The principal problems were first to determine 

 whether such embryos were composed of the same number of 

 cells that make up the normal embryo ; and secondly to see 

 whether these cells and their nuclei were of the normal size. 



An embryo that has developed from an ^^g operated upon is 

 shown in Fig. 28. The normal embryo of corresponding age 

 is drawn in Fig. 29. The first is drawn as seen from above ; 

 the second (normal) as seen in side view. Surface views of 

 the embryo showed that it is shorter and narrower from side 

 to side than the normal embryo. The cross-sections of 

 this embryo were not very satisfactory ; yet they showed two 

 important points. On one side, particularly in the posterior 

 region of the embryo, was found a layer of peculiar cells 

 lying between the ectoderm and the parablast. These cells 

 were larger than any other cells of the embryo, and very different 

 in structure. Their nuclei were very indistinct, their protoplasm 

 loose, and the cells rounded, i.e., not flattened to any extent 

 against one another. There is every reason to believe that 

 these cells have come from the protoplasm that belonged to the 

 cell removed. The nuclei have come in all probability from 

 the nuclei of the embryo. The protoplasm has been "post- 

 generated," and one will recall in this connection the descrip- 

 tion given by Roux of the regeneration of the injured side of 

 the frog's egg from wandering nuclei (cells .^) from the living 

 half. 



Another point that these sections showed is that the size of 

 the nuclei of the "half-sized" embryo is not appreciably smaller 

 than those of the normal embryo of the same stage. 



A more detailed comparison between the normal eggs and 

 eggs operated upon I have been able to make for older embryos 

 at the time when the germ-ring has just closed. 



Surface preparations show that in all cases the length of the 

 embryo coming from the eggs in which one blastomere had 

 been removed is less than normal. There is great individual 

 variation, however, in this respect. Three figures of embryos 

 are shown in Figs. 30, A, B, C. The first of these, Fig. A, is 



