2. Maturation and Fertilization. The last two cell divisions in 

 the oogenesis and spermatogenesis are known as the "maturation 

 divisions" and lead to the reduction of the chromosomes in the 

 mature egg and sperm to half the usual number. When the egg 

 is fertilized the normal number is again restored. 



Carefully study the maturation and fertilization of the egg of 

 Ascaris megalocephala, with especial reference to the chromo- 

 somes. Observe that in the maturation of the egg (also of the 

 sperm) the number of chromosomes is reduced to two, and in the 

 union of the egg and sperm the number is increased to four, the 

 normal number. Draw eggs showing all of these points. 



3. Cleavage. Observe that in the cleavage of the egg of 

 Ascaris each chromosome is split longitudinally, so that each 

 daughter nucleus receives two chromosomes from the egg and two 

 from the sperm. Draw. 



4. Development of Frog. Study entire frogs' eggs in various 

 stages of cleavage. 



Early Cleavage Stages. Examine eggs divided into two, four, 

 eight and sixteen cells. Note in the two-cell stage that a grayish or 

 slate colored area, crescentric in form, is present on one side of the 

 egg, and that it is, as a rule, bisected by the first plane of cleavage. 

 Note in the four-cell stage the relative distribution of the pigment 

 on the anterior and posterior sides of the egg. Note in the eight- 

 cell stage the relative sizes of the upper and lower cells, also the 

 distribution of the pigment in the cells, and the location of the 

 grey crescent. Note in the sixteen-cell stage the position of the 

 fourth planes of cleavage in the upper and lower cells. Examine 

 a section of one of the early cleavage stages; note the nuclei sur- 

 rounded by pigment. 



Later Cleavage Stages. Examine two of the later cleavage 

 stages (Blastula stages). Note the comparative sizes of the cells 

 in the upper and in the lower hemispheres of the embryo. Examine 

 a section of a late cleavage stage. Note the large cleavage cavity ; 

 the thinness of the roof, and especially the character of the superfi- 

 cial cells at the level of the floor of the segmentation cavity. Out of 

 these cells all of the main parts of the body develop. 



Gastrula Stages. Study surface views of three gastrula stages 

 showing : ( i ) The beginning of the dorsal lip of the blastopore, 

 (2) the back-growth of the dorsal lip, and the appearance of the 

 lateral lips, and (3) the formation of the ventral lip. At this time 

 the blastopore is circular in outline and the yolk plug fills up its 



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