THE GERM CELLS. 3 



coarsely granular than the cytoplasm of most cells, due to the presence of 

 granules or globules of yolk. These globules are suspended in the cytoplasm 

 and composed of fatty and albuminous substances that are later utilized in the 

 growth of the embryonic cells. It should be added that the composition of 

 the yolk in the human ovum is assumed, but analysis of the yolk of the hen's 

 egg has shown a large percentage of lipins including lecithin, with some pro- 

 teins also, and a similar composition of the yolk granules in other ova is a 

 reasonable assumption. Lecithin (lekythos), is a term that was used by the 

 ancients to designate the yolk of an egg. The yolk globules are congregated 

 near the center of the cell, surrounding the nucleus, while a zone of cytoplasm 

 nearly destitute of yolk forms the peripheral portion of the ovum. In his 

 recent study of the maturation of the human ovum Thomson describes and 

 illustrates a centrosphere which then disappears after the formation of the 

 second polar body. 



The nucleus is situated near but not 

 quite in the center of the ovum amidst 

 the yolk granules. Its volume bears about 

 the same ratio - to the volume of the egg 

 cytoplasm as the nuclear volume of the 

 average somatic cell bears to its cyto- 

 plasmic mass. A distinct nuclear mem- 

 brane encompasses the usual nuclear 

 structures. The chromatin seems rather 

 scanty, the nucleus thus being conspic- 

 uously vesicular. The single nudeolus 



(plasmosome) is intensely stainable, and FIG. 2. Ovumof frog(Ranasylvatica). 

 in a fresh human ovum has been observed T1 f <* ark shading represents the 



cytoplasmic pole, the light shad- 

 to perform amoeboid movement. ing immediately below represents 



The frog's egg will serve as an example sl^S^SS^^mm^e^ 



of an ovum with a moderate amount of f nts the gelatinous substance 



11- (secondary egg membrane). 



yolk suspended in the Cytoplasm, yet 



enough yolk to produce a definite and visible effect upon the organization 

 of the cell and to influence strongly the future processes of development. 

 The female frog deposits the eggs in clusters in quiet water where they may 

 be observed resting on the bottom or sticking to leaves and twigs. The 

 eggs are enclosed in a jelly-like substance, each cell with its own gelatinous 

 capsule or membrane (Fig. 2). Each egg is spherical and measures from ij^ 

 to 3 mm. in diameter, depending upon the species of frog. Externally some- 

 thing more than one-half of the cell is black owing to the presence of pig- 

 ment granules, and the remainder is nearly white. If the eggs have been 

 free in the water for a few minutes the dark sides are turned upward. A 



