444 Animal Biology 



thus perpetuating their hereditary potentialities. There seems to be a 

 relationship between the synthesis of enzymes and of new genes. It is 

 theorized that an undifferentiated cell in a particular region gives origin 

 to different and specialized cells (differentiated cells) because of cyto- 

 plasmic differences and that these differences which produce cells of 

 different sizes, shapes, and functions are the result of plasmogenes in the 

 cytoplasm. It is suggested that plasmogenes in the cytoplasm are pro- 

 duced by genes found in the nucleus. 



EMBRYOLOGY OF THE FROG 



The embryologic development of many of the animals is much the 

 same, with only minor differences in certain stages. The embryology 

 of the frog has been chosen to illustrate the general principles because 

 ( 1 ) the frog has a rather typical, representative method of development 

 and (2) the materials are usually available and rather inexpensive. 



The sexes of the frog are in different animals (diecious). The male 

 gametes (spermatozoa or sperm) produced by the male sex organs (male 

 gonads or testes) unite with the female gametes (ova or eggs) produced 

 by the female sex organs (female gonads or ovaries). This union forms 

 the first cell of the embryo known as the zygote. The eggs are found in 

 jellylike masses in fresh water pools and ponds in early spring. Each eg^ 

 has several, thick, concentric layers of jelly known as the vitelline mem- 

 brane and is about the size of a buckshot. 



In two or three hours after fertilization the zygote divides by mitosis 

 to form the two-cell stage, the two cells remaining in contact. Division 

 in this case is known as total cleavage or holoblastic (Fig. 220, A). A 

 second division occurs by mitosis in about one hour and at right angles 

 to the first plane of division, thus forming the four-cell stage. These 

 four, more or less equal, cells are called blastomeres (Fig. 220, B) . 



The next plane of cleavage is horizontal and slightly above the middle 

 or equator, thus dividing each of the four previous cells by a transverse 

 division to form the eight-cell stage. Of these eight cells, four are pig- 

 mented, smaller, located at the animal pole, and known as micromeres; 

 the other four are unpigmented, larger, located at the vegetal pole, and 

 known as macromeres (Fig. 220, C) . 



These cells continue to divide until there are a large number of cells, 

 all of which are closely packed together in a somewhat solid mass known 

 as the morula stage. The micromeres continue to divide more rapidly 

 than the macromeres at this time. It is evident that growth cannot con- 

 tinue indefinitely in this manner, or the animal would be solid without 



