CHAPTER V. 



EARLY DEVELOPMENT OF THE FROG. 



Most students have seen the eggs of the frog either in the laboratory 

 or in a pond during the springtime. They probably have observed the little 

 objects embedded in the jelly-like mass, scores of them in a cluster, each 

 egg in its own gelatinous capsule, and all the capsules clinging to one another. 

 Each ovum is a sphere, a little more than a millimeter in diameter in the 

 common wood frog and as much as 3 mm. in some other species, with a 

 dark side and a light side; and if the ovum has been at rest in its natural 

 environment for a few minutes the dark side is uppermost (Fig. 2). 



The dark color is due to the presence of brown pigment granules. The 

 portion of the egg where there is less pigment contains an abundance of yolk 

 globules suspended in the cytoplasm, while the darker part consists of 

 cytoplasm with fewer yolk globules. The nucleus of the cell is located in 

 the part containing the more cytoplasm 

 and is therefore eccentric. The distribu- 

 tion of cytoplasm, yolk and pigment is 

 apparently an expression of the internal 

 organization of the egg, yielding here a 

 visible polarity. The cytoplasmic or 

 ,aiiimal pole contains the nucleus and 

 abundant pigment, the latter mostly near 

 the surface; the yolk or vegetal pole con- 

 tains less cytoplasm and pigment but 

 abundant deutoplasm (Fig. 26). As far 

 as determined, the egg is radially sym- 

 metrical around the axis extending from 

 the center of the animal pole to the center 

 of the vegetal pole; that is, assuming this 

 axis to be vertical, the egg possesses the 

 same organization in all radii drawn from 

 the axis in any given horizontal plane. 

 The polarity and symmetry _of the egg are 

 important factors in development. 



The eggs are expelled by the female frog into the water and the sper- 

 matozoa discharged by the male mingle with the egg clusters. A sperm 

 4 49 



FIG. 



26. Section through the fully 

 formed ovarian egg of a frog. 

 Morgan. The protoplasmic or 

 animal pole is toward the top of 

 the page. Note that the nucleus 

 is situated nearer the animal pole, 

 that is, in the center of the cyto- 

 plasmic mass. The yolk globules 

 can be seen in the lower part of 

 the figure. 



