19° BULLETIN OF THE BUREAU OF FISHERIES. 



The pronuclei move toward each other, fuse, and disappear during the following 

 10 to 15 minutes. 



CLEAVAGE AND FORMATION OF GERM LAYERS. 



The pear shape of the cell becomes more pronounced, and one side of the broad end 

 increases more rapidly in size than does the opposite side (fig. 181 Tc, p. 188). At the same 

 time the cytoplasm becomes less dense at the vegetative pole, which permits more light 

 to pass through and therefore makes this region appear brighter and less granular than 

 the rest of the egg. 



One or two minutes later the opposite side of the egg bulges rapidly, making the 

 pear shape more symmetrical, as is shown in figure 181/ (p. 188). The sides continue to 

 protude outward, a depression appears at the formative pole, and a constriction begins 

 to form about the proximal end of the nutritive pole (fig. 182a). Then two planes of 

 constriction appear as shown in figure 1826 and, to all external appearances, a body 

 of three nearly equal cells results. This, however, is really not the case, for during the 

 next 10 minutes radical changes occur in the egg. One of the apparent cells fuses with 

 the one that corresponds to the nutritive pole, as shown in figure 182, c to e, resulting 

 ultimately in two cells of very unequal size. The large one is known as the macromere, 

 and the small one as the micromere. The nuclei are visible as clear round spots in the 

 center of the two cells. During the next 10 minutes two more micromeres result. The 

 first one of these is given off from the macromere, and almost at the same time the 

 second one results from the division of the original micromere, as shown in figure 182,/ 

 and g. 



From this point on, cell multiplication through the division of the micromeres and 

 the giving off of new micromeres from the macromere is very rapid. The stages repre- 

 sented in figure 182, h to ;', are passed through in about half an hour. The result is a 

 macromere almost covered with a cap of micromeres and a small segmentation cavity 

 inclosed by the group of cells. The relation of the cells to the segmentation cavity at 

 this stage is best seen in an optical section represented in figure 182/fe. 



As cell division proceeds, the ectodermal cells become smaller and ultimately com- 

 pletely envelop the macromere, which finally divides into two equal cells (fig. 182/). 

 These two cells are apparently the forerunners of the mesoderm. 



Figure 183a (p. 193) shows a later stage where the embryonic cells have become 

 more uniform in size. The first polar body has, in the meantime, divided so that three 

 polar cells are visible at this stage. Very fine cilia develop on the exposed surface of 

 the cells at this time, and by their vibrations cause the embryo to move slowly about. 

 The period of development from fertilization to the free swimming embryo requires 

 normally 4K to 5 hours. 



Internal changes are difficult to follow in the living material from this stage on. 

 Up to the end of the twenty-fourth hour the principal changes observed are the rapid 

 multiplication of cells, the extraordinary growth of cilia over the outer surface of the 

 body, and the development of a long flexible flagellum, which is composed of several 

 filaments. This flagellum is situated on the anterior end of the body and is held forward 

 like an antenna as the trochophore propels itself rapidly through the water by means 

 of the cilia (fig. 183c, p. 193).. 



