186 GERM-CELL CYCLE IN ANIMALS 



Neritina which finally reach the velar cells. It is 

 also probable that Fol (1880) observed similar gran- 

 ules in the 16-cell stage of Planorbis. In the same 

 category, no doubt, belong the bodies figured by 

 Fujita (1904) in the 4-cell to the 16-cell stages of 

 Siphonaria lying at the vegetative pole, and the 

 "Ectosomen" described and figured by Wierzejski 

 (1906) in Physa. These granules appear at the vege- 

 tal pole in the blastomeres of Physa during the 

 second cleavage ; are at first embedded in the ento- 

 derm mother cells, but finally become localized in 

 the ectoderm cells. They periodically appear and 

 disappear, and may, as suggested by Wierzejski, 

 represent only "eine besondere Erscheinung des 

 Stoffwechsels" (p. 536). 



Similarly in the rotifer, Asplanchna, Jennings (1896) 

 has traced a "cloud of granules" from the eight-cell 

 stage until the seventh cleavage, when this mass 

 forms part of the smaller entodermal cell. In Lepas 

 there has also been recorded (Bigelow, 1902) a segre- 

 gation of granules in one blastomere. Many other 

 substances granular in form have been described in 

 the eggs of animals, some of them at least having 

 migrated there from the somatic tissue. Blockmann 

 (1887) discovered a number of bacteria-like rods 

 in the undeveloped eggs of Blatta germanica; these 

 rods multiplied by division and were considered sym- 

 biotic bacteria. " Bacterienartige Stabchen" were 

 also noted by Heymons (1895) in the eggs of Peri- 

 planata orientalis and Ectobia livida; these sink into 

 the yolk and disappear. More recently a report of 



