368 THE NEW INDIVIDUAL Part IV 



the ponds, their white vegetal poles are turned toward the dark bottom and 

 their black poles toward the light. This is due to relative weight but it results 

 in excellent concealing coloration. Yolk supplies the tadpoles with food until 

 they are well beyond hatching. Embryos have the same general needs as frogs, 

 plenty of water and food, income of oxygen, and outgo of carbon dioxide, 

 water and urea. They are easily killed from the by-products of their own metab- 

 olism and are so sensitive to temperature that they will reach any given stage 

 of development almost three times faster at 20° C. than at 10° C. 



Reproduction Ends — Development Begins. Reproduction ends with two 

 processes that are extremely important to the new individual. They are: (1) 

 the maturation of the sex cells whereby their chromosomes are reduced to 

 half the number in the body cells; and (2) fertilization with its immediate 

 effects upon the organization of the egg, followed by the union of the sperm 

 and egg nuclei and the reestablishment of the whole number of chromosomes 

 (Fig. 19.6). 



The entrance of the sperm always occurs in the hemisphere of the animal 

 pole and stimulates a reorganization of the egg which makes it repellent to 

 other sperm cells. Even if the egg membranes have been removed, a sperm 

 will not enter a fertilized egg. As before mentioned, experiments have proved 

 that the reorganization and development of an egg can be stimulated by vari- 

 ous shocks, pricks, solutions, and shakings. Frogs have grown to young adult- 

 hood with only pricks and chemical solutions for fathers. 



Among the results of the reorganization is the gray crescent, an area oppo- 

 site the entrance point of the sperm, from which some of the black pigment 

 retreats. Staining parts of the egg has shown that a plane that passes through 

 the axis of the egg and bisects the gray crescent usually divides the future ani- 

 mal into right and left halves. Since the first cleavage plane bisects the gray 

 crescent it follows that the bilateral symmetry of the embryo is prearranged in 

 the egg. 



Cleavage. Successive cell divisions follow one another at intervals of about 

 an hour varying with the temperature. The speed with which new cell mem- 

 branes grow is slowed down as the membrane formation plows through the 

 yolk. In the animal hemisphere, the wall of the blastocoel is thin because the 

 cells contain so little yolk; in the vegetal pole it is thick because they contain 

 so much (Fig. 19.7). 



Within 12 hours after fertilization (at 18° C.) the embryo, usually in the 

 late blastula stage, contains hundreds of cells. The speed with which they multi- 

 ply makes it hard to realize that with every division a nucleus with its thou- 

 sands of genes is accurately allotted to each daughter cell. Equal distribution 

 of parental genes begins with the first cell division and is repeated through 

 billions of divisions in the growth of animals from jellyfishes to man. 



Gastrulation. Gastrulation proper in amphioxus, for example, includes only 



