48 LABORATORY STUDIES IN ZOOLOGY 



position in the cytoplasm of the egg. Note the head containing 

 the round nucleus and the more pointed portion termed the 

 refractive body. Also the lack of an enclosing capsule about the 

 egg. In some of the eggs the refractive body of the sperm is 

 probably being absorbed. Draw. 



First Maturation Division of the Egg (Row 2). — This is] known 

 as reductional division, as the number of chromosomes of the egg 

 are reduced by one-half so that the number of chromosomes 

 peculiar to the body cells of the species remain constant after 

 un'on with the sperm nucleus (pronucleus) which has undergone 

 reductional division of its chromosomes before entering the egg. 

 Find an egg showing a spindle upon which are two sets of chromo- 

 somes. One set will be separated off and thrown out of the 

 cytoplasm of the egg as a small discarded cell known as the first 

 polar body, the other set will be retained as those of the egg 

 nucleus. The sperm pronucleus ought to be at rest somewhere 

 within the cytoplasm. Draw. 



Second Maturation Division of the Egg (Row 3). — This is 

 known as equational division, as the number of chromosomes of the 

 egg nucleus remains the same thereafter and is a true mitotic 

 division resulting in a second polar body which is cast off and, 

 like the first, comes to naught. In a typical example of this 

 stage the first polar body ought to be seen clinging to the inside 

 of the egg capsule, the chromosomes of the second polar body on a 

 spindle with those of the egg mucleus, and the sperm pronucleus 

 at rest somewhere in the cytoplasm of the egg. Note that the 

 cytoplasm of the egg has withdrawn from the egg capsule leaving 

 a space between the two. The cell remaining after the separa- 

 tion of the two polar bodies becomes the functional egg. Draw. 



Stage Preceding the Union of Male and Female Pronuclei 

 (Row 4). — This might be compared with the resting stage during 

 mitotic division of somatic cells, since mitotic division follows 

 the union of the male and female pronuclei. Look for a cell 

 which shows : (a) two nuclei, one the male, the other the female, 

 but indistinguishable from one another; (6) the first polar body 

 within the egg capsule, and the second polar body clinging to the 

 outer margin of the cytoplasm of the egg. Draw. 



A STUDY IN DEVELOPMENT 



The object of this study is that of illustrating the manner in 

 which a multicellular animal is derived from a single cell. All 



