42 THE FROG 



IV. REPRODUCTION AND DEVELOPMENT 

 A. Related Cell Activitles 

 Exercise 39. — Cell Division. 



(a) The di%'ision of the cjtosome of a cell is preceded by division 

 of its nucleus. Although nuclear division occurs by two different 

 methods, amitosis and mitosis, the latter method is almost universal. 

 The life-cycle of a cell is divided into phases for purposes of discussion. 

 Periods characterized by metabolism and growth, during which the 

 cell is said to be in the metabolic or vegetative phase, alternate with 

 periods of division during each of which the dividing nucleus of the 

 cell passes through the prophase, metaphase, anaphase, and telophase 

 and the cjtosome is separated into two parts iFig. 2S). For the study 

 outlined here, sections of growing onion, hyacinth, or may-apple root 

 tips, or of the epithelium of salamander larvae can be used. Examine 

 the sections with tlie low-power objective and understand the relation 

 of the parts. Then, with the high-power objective, observe the parts 

 of the cells in the vegetatiAT phase. Find cells in the different phases 

 of division and note the changes which occur as the mitotic spindle 

 and chromosomes are formed, as the half-chromosomes separate, and 

 as the cjtosome divides. Can you determine how many chromosomes 

 are present in each cell? 



(b) Draw (2 or 3 X the measured size as projected to table level) 

 the outlines for six cells. Label these outlines vegetative phase, early 

 prophase, middle prophase, metaphase, anaphase, and telophase. As 

 you find and identify a cell in each of these stages add tlie details of 

 its structure to the correct cell outline. 



Exercise 40. — Differentiation of Germ Cells. 



(c) Examine demonstrations showing primary spermatocytes or 

 primary oocytes and the two meiotic divisions. Understand the 

 significance of the disjunction of the members of homologous pairs of 

 chromosomes which occurs at this time (Fig. 29). The eggs of the 

 nematode Ascaris equonim {A. megalocephala bivalens) are excellent 

 material for this exercise, but many other forms can be used since 

 the phenomena are universal among animals. 



B. The Germ Cells and Fertilization 

 Exercise 41. — The Germ Cells of the Female Frog. 



(a) Examine in a watch glass of water a small mass of eggs from 

 the ovan.' of a frog which has been preserved in formalin. Look with 

 the handlens and the low-power objective of the compoimd microscope 



