48 THE FROG 



(d) Since egg-laying occurs only in the spring the following direc- 

 tions have been written with reference to preserved material. In 

 studying this dead material, examine not only the exact stages speci- 

 fied but also look for intermediate conditions. Wherever possible 

 arrange specimens in a series showing the transition from one stage to 

 another, and thus obtain a concept of development which approaches 

 that obtained by the study of living eggs and embryos. Reference 

 can be made to Fig. 31 for the general sequence of early development. 



Exercise 46. — The Two-cell Stage. 



(e) After amphimixis has occurred, the nucleus of the zygote 

 enters the prophase of mitosis. When the telophase is reached a con- 

 striction appears at the animal pole and rapidly passes toward the 

 vegetal pole, dividing the zygote into two cells. This division is 

 known as the first cleavage and the constriction is known as the first 

 cleavage furrow. In this and subsequent stages the cleavage furrow 

 indicates that a cell division following mitotic nuclear division is com- 

 plete. During cleavage the vegetative phase is relatively short and 

 divisions succeed one another at intervals that depend somewhat upon 

 the temperature. In preserved material the two-cell stage is best 

 studied in an egg with the cleavage furrow encircling about two-thirds 

 of the circumference. Examine several specimens under water in a 

 watch glass with the handlens and the low-power objective of the 

 compound microscope. Draw one from side view to show the first 

 furrow in process of formation or as it appears when completed. This 

 and subsequent drawings should be made X 20, in order that change 

 in size as development continues can be shown. Indicate the polar 

 axis by an arrow as in Exercise 44 and number the furrow. Do not 

 represent the jelly or pigment in this or later drawings unless specified. 



Exercise 47. — The Four-cell Stage, 



(f) Examine several preserved specimens of eggs in which the 

 second cleavage furrow is well advanced. What has happened inter- 

 nally before this furrow appears on the surface? The two nuclei 

 divide at the same rate, and the cytosomal constrictions appear in 

 both cells at the same time and together are known as a single cleav- 

 age furrow. Draw from the animal pole, numbering the furrows 1 

 and 2, and the poles as before. 



Exercise 48.— The Eight-cell Stage. 



(g) The third cleavage furrow is horizontal. How is it placed 

 with reference to the e(iuator of the sphere? With its completion the 



