158 ELEMENTARY BIOLOGY. [CHAP. 



i. Segmentation of the fertilized ovum (oosperm) and 

 larval metamorphosis. 



The rate of both segmentation and development 

 generally is liable to vary, in accordance with cir- 

 cumstances. The periods enumerated below are 

 approximately those best suited for observation. 



The specimens should be examined both in the 

 fresh state and when fully preserved. 



a. One hour after fertilization. 



a. General characters ; shape and size ; the black 

 pigmented upper pole ; the yellowish white non- 

 pigmented lower pole. 



/?. The first cleavage furrow ; an annular constric- 

 tion passing right round the egg, longitudinally 

 with respect to the afore-named poles. It con- 

 stricts the whole into two equal halves (embryo 

 cells or blastomeres). 



b. Two hours after fertilization. 



a. The second cleavage furrow ; longitudinal and at 

 right angles to the first one, subdividing each 

 blastomere into two. There are now four cells 

 of equal size. 



c. The same at 3 4 hours. 



a. The third furrow ; at right angles to the two 

 former and excentric, lying within the pig- 

 mented pole. It subdivides the whole into four 

 smaller pigmented upper layer cells^ and four 

 larger non-pigmented lower layer cells. Ex- 

 amine from several aspects. 



d. The same at 6 8 hours. 



A mulberry-like mass conspicuous by the irregu- 

 larity in size of the blastomeres. 



