50 THE FROG 



Exercise 49. — The Twelve-cell and Sixteen-eell Stages. 



(h) When the eight cells divide it is obvious that the cytosomal 

 constrictions in all of them cannot be in the same plane. Actually 

 they appear in two vertical planes, so that we say there are two fourth 

 cleavage furrows. The four upper cells, which contain less yolk than 

 the lower cells, often complete their division before the furrows have 

 appeared in the lower hemisphere. A twelve-cell stage results, with 

 eight smaller cells toward the animal pole and four larger cells below. 

 With the division of the four lower cells the sixteen-cell stage is 

 produced. Examine eggs preserved in this stage and determine the 

 exact outlines of the cells at each pole in several specimens. What 

 forces determine the cleavage pattern at this stage? Draw a twelve- 

 cell or a sixteen-cell stage from the animal pole, reproducing carefully 

 the exact cell outlines of a typical specimen; number the furrows as 

 accurately as possible. 



Exercise 50. — The Thirty-two-cell Stage. 



(i) The cytosomal constrictions appear in the division of the six- 

 teen cells in such a way that there are two fifth cleavage furrows 

 which are horizontal, one above and one below the third cleavage 

 furrow. With the completion of these furrows, we have a thirty- 

 two-cell stage. Why is it difficult, in studying any single preserved 

 specimen of this stage, to assign numbers to the different furrows? 

 A thirty-two-cell stage is a theoretical rather than an actual occur- 

 rence, because the cells about the animal pole now divide somewhat 

 faster than the yolk-laden cells of the vegetal hemisphere. Why is 

 there this difference in the rate of division? In the living egg the 

 number of cells continues to increase by successive cell divisions, but 

 from this time on it is impossible to recognize any uniformity in the 

 pattern made by the furrows. Make a diagram of a thirty-two-cell 

 stage from the side view and number the furrows. Draw a side view 

 showing the exact appearance of an egg which is in about this stage 

 of development. 



D. The Blastula Stage 



Exercise 51. — The Early Blastula. 



(a) The thirty-two-cell stage is sometimes called the early blastula. 

 More and more of the stored yolk is utilized in continued cell division, 

 and the resulting cleavage cavity, or blastula cavity, becomes con- 

 spicuous internally. Take a preserved specimen in an early blastula 

 stage and, after removing the jelly by rolling on damp filter paper, 

 divide it into halves by a vertical sliding cut with a sharp scalpel or 



