86 THE BIOLOGY OF THE FROG , chap. 



fluid known as nuclear sap, but just preceding the escape of 

 tlie egg into the body cavity and during its passage down 

 the oviduct the nucleus becomes shrunken through the exu- 

 dation of its fluid, and undergoes a process of division in- 

 volved in the formation of \}i\t first polar body. As Schultze 

 has shown, along with the shrinkage of the nucleus there 

 appears a mass of fluid beneath the animal pole which he 

 considers to be the nuclear sap. The place is marked by a 

 light-colored spot near the center of the dark cap. The 

 great mass of the frog's egg is made up of yolk, which 

 occurs in the form of granules embedded in the cytoplasm. 

 The yolk is a semi-fluid albuminous substance which is em- 

 ployed for the nutrition of the developing embryo. It is 

 more abundant tpward the light-colored or vegetal pole of 

 the egg, the region around the animal pole containing rela- 

 tively more cytoplasm. The yolk granules are of various 

 sizes, and are usually spherical or oval in form. By the 

 action of certain reagents they may be broken up into flat- 

 tened plates which, according to Schultze, do not exist 

 as such in the living egg. 



Maturation. — The process of maturation consists in two 

 successive divisions of the ovum, resulting in the formation of 

 the two polar bodies. These bodies are minute globules 

 extruded at the animal pole of the egg. Morphologically they 

 are cells, produced by very unequal divisions of the egg, 

 which is only a cell of very large size. The first polar 

 globule is given off while the egg is within the body, the pre- 

 liminary steps of the process occurring just before the egg 

 leaves the ovary. The large watery nucleus shrinks, the 

 chromatin becomes aggregated into definite bodies or chro- 

 mosomes, a spindle forms at right angles to the surfaceof the 

 egg, and half the chromatin, with a small amount of cyto- 

 plasm, is extruded as the first polar body. 



