REPRODUCTION AND DEVELOPMENT 45 



seen in this material. They are single cells like the smaller ones but 

 much yolk has been stored in the cytosome of each during the growth 

 jicriod of oogenesis which precedes the formation of the polar bodies. 

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

 more of the smaller oocytes, showing the parts in outline only. 

 Examine a permanently mounted and stained section of the ovary 

 of a young frog. This will show the follicle cells surrounding each 

 oocyte; add these to your drawing. 



(b) Remove a single large oocyte from the ovary of a recently 

 killed frog, crush on a slide under a cover glass, and examine with 

 the high-power objective. What can you make out about the physical 

 nature of the protoplasm? Observe the numerous yolk bodies. Is 

 there any indication of the preformation of a new individual? 



Exercise 42.— The Germ Cells of the Male Frog. 



(c) The mature male germ cells, or spermatozoa, can be secured 

 by cutting a testis from a freshly killed frog into small bits and teas- 

 ing in water. Examine with the compound microscope and look for 

 elongated bodies. Do these move about? Can you see how? Each 

 spermatozoon consists of an enlarged portion or head, containing the 

 nucleus, and the mid-piece from which extends the elongated thread- 

 like flagellum, or tail. These parts are distinguished more easily after 

 the addition of iodine. Like the female germ cell, the spermatozoon 

 is a single cell (Fig. 30). Draw one or more spermatozoa (10 X 

 measured size projected to table level). 



Exercise 43. — Activation and Amphimixis. 



(d) Examine demonstrations of favorable material showing the 

 entrance of the spermatozoon into the female germ cell with its conse- 

 quent activation. Activation is indicated by the lifting of the fertiliza- 

 tion membrane, the initiation or completion of the meiotic divisions in 

 many forms, and by the beginning of cell division, or cleavage. In 

 normal fertilization amphimixis also follows the entrance of the sperm. 

 During this process the two haploid sets of chromosomes that are pres- 

 ent in the male and female pronuclei, or nuclei of the fully differenti- 

 ated gametes, unite to give rise to the diploid set of chromosomes in 

 the zygote. 



Exercise 44. — The Fertilized Egg, or Zygote, of the Frog. 



(e) In the frog the female germ cell is ovulated, or freed from the 

 ovary, as a secondary oocyte, that is, after the first meiotic division 

 has occurred. How does it reach the oviduct? As the eggs pass along 

 the oviduct, layers of the gelatinous secretion of the cells of the ovi- 



