462 MORGAN. [Vol. X. 



APPENDIX. 



The preceding pages were finished and sent to the editor early in 

 tlie spring of 1894. During the summer of '94 I was able to con- 

 tinue my study of teleostean development at the Biologische Anstalt 

 in Helgoland. I wish to express here my indebtedness to the 

 Director and Staff of the station for many courtesies extended to me 

 during my sojourn in Helgoland. 



It is not my intention to enter here into a full account of the 

 results of this renewed study, but there are two not unimportant 

 additions that I can make to the preceding pages. My account 



of the outermost layer ("covering layer") of the extra-embryonic 

 region, embryo, and germ-ring, is very imperfect, owing to the diffi- 

 culty of recognizing this layer in the older embryos. The layer be- 

 comes exceedingly thin in the later stages, and in sections forms only 

 a thin membrane over the other cells. I found during the summer 

 of '94 that if the eggs of Ctenolabrus were killed by weak osmic 

 acid, then thoroughly washed in distilled water, placed in a solution 

 of I % silver nitrate until they were made brown, washed, and 

 opened in dilute glycerine, very beautiful surface preparations could 

 be obtained. 



Without figures a detailed account of these preparations would be 

 unprofitable. Only the cells of the outermost layer ("covering 

 layer") are stained by this method. It is found that these cells, 

 individually, increase very much in surface area as the blastoderm 

 grows over the yolk. Moreover, these cells divide during the same 

 period, as the shape of the outlines of many of these cells shows. We 

 often find pairs of rounded cells indicating that cell-division has just 

 been finished at these points. In other preparations, where a differ- 

 ent method has been used (chromic acid), the karyokinetic figures 

 themselves are sometimes seen in the cells of this layer. 



I have not yet calculated whether the increased area of each cell 

 will, when all the cells are taken together, account for the increas- 

 ingly larger area covered. This is probably true, however, because 

 there is no evident source from which new cells could arise. 



Stained preparations were made to see whether karyokinetic divi- 

 sion takes place in the second or inner layer of the extra embryonic 

 region, and germ-ring. Cells in process of division are at all times to 

 be found in this hmer layer. Especially numerous are karyokinetic 

 figures in the tail-knob of the embryo. Many are also found in the 



