236 



AYERS ON THE DEVELOPMENT 



the yolk and at once begin to assimilate the yolk matter, changing it into cell protoplasm 

 and nuclear substance. They increase rapidly by division in a manner similar to the 

 blastoderm cells, but owing to their high degree of nuclear development they exhibit but 

 a thin covering of protoplasm. The amoeboid processes * from each cell extend out among 

 the yolk spheres and coalesce with neighboring pseudopodia, so that when the number 

 of such cells becomes quite large there is fully formed an intricate network of proto- 

 plasmic filaments such as is 

 partially indicated in Figs. 

 18 - 20. The nuclei appar- 

 ently control this network and 

 at times draw to themselves 

 their pseudopodia, sending out 

 new ones from other parts 



of the surface to unite with v ..» ■» J ~ y >n\ Vfec-r-f 

 some other portions of the 

 common network or to en- 

 gulf and feed upon the yolk 



Fig. 18. Surface view of the serosa cells in an uninjured egg of Oecantlms. The pro- 

 toplasm of the cells radiates from the central nucleus. X about 150. 



Fig. 19. Optical section of the protoplasmic network in an egg of Oecanfhus. X 300. 



globules. When a free amoe- 

 boid cell comes in contact with 

 a yolk globule, it folds or 



creeps over it and, in case the yolk body is large as com- 

 pared with the cell, surrounds it by a thin layer of clear 

 protoplasm, the nucleus of which bulges out from the side 

 of the globule but still remains entirely surrounded by a 

 thin layer of cell substance. In sections of hardened and 

 stained eggs these amoeboid cells remind one of certain 

 rhizopods that possess anastomosing filamentous pseudo- 

 podia extending in a radial manner from the entire 

 surface of the central mass. In these amoeboid cells the 

 fine threads of protoplasm are comparable with pseudo- 

 podia, and extend in all directions from the protoplasm 

 which surrounds the central nucleus. The amoeboid cells 

 which reach the periphery form a thin continuous cell layer 

 of uniform thickness over the whole surface of the yolk, — 

 the so-called blastoderm. 2 



A tract of the blastoderm along the median line of the 

 ventral (concave) side, lying nearest the deep or primitively 

 head end of the egg, becomes thickened into a germinal 

 band, which is the first trace of the body of the embryo. 

 This thickening is caused by the proliferation and elonga- 

 tion of the flattened cells of the blastoderm in this region. 

 On sections of this stage the germinal band is seen to be composed of a single layer of 



20. 



Surface view of the edge of the body wall a 

 it is advancing over the yolk. From the em" 

 bryo figured in pi. 19, fig. 1, in the region 

 crossed by the line b. The space between the 

 body wall and the line ;, which limits the ventral 

 extent of the yolk, is seen to be more or less 

 covered with amoeboid cells connecting the 

 yolk area with the edge of the body wall. 

 The yolk mass lies underneath the entire 

 field." X 800. 



1 Compare Graber (22). 



2 The " Keimhautblastem " is entirely wanting. 



