Chap. 19 



DEVELOPMENT 

 DEUTOPLASM 



363 



Fig. 19.2. Fertilization in the guinea pig. Microscopic sections of eggs taken be- 

 fore and soon after mating. The eggs are minute, smaller than fig-seeds. Deuto- 

 plasm is protoplasm that is permeated with particles of yolk. The yolk may be light 

 (A) or dark colored (B) depending partly upon the stain used in its preparation. 

 A, before fertilization. The first maturation division with the nucleus now in meta- 

 phase. This results in two cells, the egg with half its former number of chromo- 

 somes and the first polar body {/ P.B.), a minute cell that contains the other half. 

 The egg is enclosed in a special (vitelline) membrane. B, preparation for fertiliza- 

 tion. The sperm has just entered the egg. The first polar body (/ P.B.) is outside 

 the egg cell. The entrance of the sperm stimulates the completion of a second di- 

 vision (// P.B.). Changes in the position of particles show that the sperm affects 

 the whole egg. C, fertilization. The nuclei of egg ( 9 ) and sperm ( $ ) are almost in 

 contact. Each one has half the number of chromosomes that is characteristic of the 

 body cells of a guinea pig. The polar bodies are disintegrating outside the egg. 

 (Courtesy, Nelsen: Comparative Embryology of the Vertebrates. New York, The 

 Blakiston Co., 1953.) 



chord and neural plate; the gray yolk will be the inner layers of cells or endo- 

 derm, and the remainder of the egg will become the outermost cells or skin 

 ectoderm. The identity of parts of amphibian embryos has been followed by 

 coloring them and tracing their future careers in the animal, and also by trans- 

 planting them to other regions to test the effect of changed locations. Both 

 methods are widely used in experimental embryology. 



Development of the Lancelet 



A lancelet is a transparent fishlike animal about three inches long. Its 

 lance-shaped tail gave it the name lancelet and its sharp-edged body the name 

 amphioxus (double edged) (Fig. 19.4). There are about two dozen species of 

 Amphioxus distributed in the warmer seas over the world, including those 

 along the southeastern and -western coasts of the United States. Lancelets from 

 the Bay of Naples have long been known to biologists and those from the 

 waters near Amoy, China, to fishermen who may harvest as much as a ton of 

 them per day. In the breeding season, the males and females leave the sand 

 and swarm in shallow water. Eggs and sperm are shed in the open water and 

 fertilization occurs there. At this time the cytoplasm of the egg is apportioned 

 out for particular destinations in the embryo. 



The eggs are just visible to the naked eye (0.1 mm. diameter) and contain 



