OBSERVATIONS ON DEVELOPMENT OF COMMON TROtJT. 117 



the process of segmentation, extends as a thin crust on the 

 surface of the yolk outside the saucer-like depression, and, as 

 will be shown afterwards, is important for the development 

 of the layers of the embryo and some of its tissues. Oellacher,^^ 

 who first correctly described (1. c, p. 12) that part which 

 corresponds to our parablast — but who unfortunately calls it 

 vitelline membrane, not knowing its great importance for the 

 future development of the embryo and embryo-cells — mentions 

 that it is a direct continuation of the germ, already before the 

 germ undergoes segmentation. Although I have not been 

 able to ascertain how far the parablast extends on the yolk- 

 sphere, still I am inclined to accept Oellacher's very tenable 

 proposition, that the ovum of trout is comparable to a fat 

 cell, thus : the protoplasmic mantle of the fat cell consists of 

 a thickened portion which includes the nucleus, and of the 

 rest which is only a thin crust of protoplasm ; the thickened 

 nucleated part corresponds to the unsegmented archiblast or 

 blastoderm TAuct.) with its germinal vesicle, while the rest 

 of the mantle is represented by the parablast ; to the oil drop 

 of the fat cell included in that protoplasmic mantle, corresponds 

 the yolk (food-yolk) of the ovum. 



[Vogt^ speaks of a vitelline membrane in the ovum of 

 Coregonus palcea, which in the fertilized ovum is continuous 

 with the blastoderm. He says, p, 29 : " Le renflement 

 (blastoderma Auct.) occupe invariablement le milieu du 

 disque huileux . . . . et ses bords passent insensiblement a 

 la membrane vitillaire qui a Fair de le recouvrir." 



LerebouUet^ (p. 127) says that an ovum which imme- 

 diately after fecundation is coagulated by acidulated water 

 contains an amorphous, granular, membranous pellicle which 

 surrounds a third or half of the ovum, and includes also oil 

 drops ; it lines also the saucer-like depression and represents 

 the future " feuillet organique " or " feuillet muqueux " of 

 the embryonal germ,*, e. of the blastoderm, Auct., which lies 

 above it, and which is very adherent to that pellicle. The 

 " feuillet muqueux " does not participate in the segmentation 

 (I.e., p. 128); it consists (1. c, p. 134) of a thickened peri- 

 phery and a thin central lamella ; while the blastoderm grows 

 around the yolk it is accompanied by the "feuillet muqueux," 

 which precedes, however, the former (1. c, p. 136). Later 

 on the ''" feuillet muqueux " contains also cells, and enters 

 into the formation of the wall of the intestine. 



Kupfer^ (p. 217) mentions the appearance of nucleated 

 cells around the blastoderm on the surface of the yolk in the 

 ovum of Gasterosteus and Spinachia at a somewhat late 

 stage of segmentation. These cells are not derived from the 



