206 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the surface, as in fig. 238, but this must not be mistaken for one of the phagocytes or 

 yolk cells, which are formed, in an entirely different manner. 



Karyokinetic figures, which are clearly seen in all dividing nuclei, show that up 

 to this time the plane of cell division is radial, for all cells at least which have 

 approached as near the surface as those shown in fig. 242. A little later than this, fig. 

 224, plate 50, and fig. 244, plate 52, when over 200 cells were present, phagocytes or 

 yolk cells suddenly appear. One egg showed, when sectioned, the following condition : 



Number of cells at surface 219 



Number of yolk cells 11 



Total number of cells iu egg 230 



Number of cells in active karyokinesis: 



Radial division 15 



Tangential division . . 2 



I » 



The yolk cells are in various stages of migration from the surface to the depths 

 of the egg. Four have reached points about midway between the surface and center, 

 and one of these is in the metakinetic stage of division. They originate by the 

 tangential division of a certain number of the peripheral cells and by the multiplication 

 of yolk cells thus formed. The peripheral cells at this time are not quite in contact 

 with the surface of the egg, but are separated i'rom it by a thin layer of yolk spherules. 

 When a peripheral cell divides in a horizontal or tangential plane, the central daughter 

 cell migrates into the great yolk mass, filling the egg, while the other remains at the 

 surface and forms a part of the primitive blastoderm. The process is illustrated in 

 figs. 236 and 243, the latter showing the formation of a yolk cell near the surface, and 

 also the division of a yolk cell already formed. 



THE INVAGINATION STAGE. 



After a stage similar to that shown in fig. 225 is reached the peripheral cells continue 

 to divide in radial planes, and their protoplasm soon bounds the surface of the egg. 

 Cell division becomes more rapid over one side of the egg, possibly that corresponding 

 to the animal pole, but this was not finally determined. An area of rapid proliferation 

 is thus distinctly marked off, and in the midst an invagination of superficial cells 

 occurs. This begins by the in-wandering of a few cells, which is followed by the multi- 

 plication of those thus immersed in the common food stock, and by the sinking in of a 

 small area of the blastoderm about this point. In an ovate egg, like that shown in 

 fig. 227, the invaginate area lies toward one of the poles. 



The depression is at first very shallow, but increases considerably in depth and 

 becomes a well-defined circular pit. Later it elongates transversely (cut 26), and in 

 abnormal cases a deep gutter is formed. The character of this depression is indeed 

 subject to great variation. The pit at the surface lasts from four to five days, when, 

 after becoming elongated into a slightly triangular slit, by the ingrowth of the sides, 

 it completely disappears. 



In an egg in which there is a distinct patch of cells marking the area of invagina- 

 tion, but where the depression is very slight or has entirely disappeared, the embry- 

 onic area which lies in front of the point of invagination is marked by a wonderful 

 activity among the superficial cells. This is illustrated in fig. 252, plate 54. About 

 the point of invagination there is a mass of several hundred cells, from which migra- 

 tion into the yolk has taken place. Many of the cells, both at and below the surface, 



