564 



CONTINUITY OF LIFE 



obvious difficulties in obtaining suitable 

 material we must resort to lower animals. 

 Furthermore, human development is com- 

 plicated by distinct specializations which 

 are difficult for the beginner to understand. 

 Obviously, much less is known about it, 

 as well. Because the development is rela- 

 tively simple and yet typical, we shall select 

 the eggs of Aniphioxus, a primitive chor- 

 date, for our study (Fig. 23-2). Eggs of 

 starfish and sea urchins also provide ex- 

 cellent material and are frequently used for 

 this study (Fig. 23-3). 



Cleavage 



In some animals the sperm enters the egg 

 before the latter is completely mature, that 

 is, before the formation of the second or 

 even the first polar bodies. This is usually 

 the case in vertebrates and is true of Am- 

 phioxus. The first sperm that strikes the 

 membrane surrounding the egg enters head 

 first, dropping its tail the moment it is in- 

 side. It enlarges to become the male pro- 

 nucleus and then lies at rest in the cyto- 

 plasm while the egg nucleus completes its 

 last division. When the egg nucleus is ma- 

 ture, the two pronuclei unite and the first 

 division of the zygote follows shortly. In 

 frogs this occurs in about one hour. The 

 split divides the zygote into two equal 

 parts. Cleavage divisions follow in rapid 

 succession, resulting in a hollow ball of 

 cells, the total being no larger than the 

 original zygote. All that has happened is 

 that the food-laden cytoplasm of the egg 

 has been sacrificed to produce many cells, 

 each containing a nucleus and cytoplasm. 

 Growth, which is a different process, has 

 not occurred. Each cell has the potentiali- 

 ties of producing other cells and eventually 

 tissues and organs. Such a hollow ball of 

 cells is called a blastula (Fig. 23-2). 



Formation of germ layers 



The ball of cells indents on one side until 

 the two layers of cells almost touch, giving 

 the appearance, somewhat, of a rubber ball 



which has been pushed in with the finger. 

 This is the gastrula, and the single opening 

 formed at one end is the blastopore. The 

 newly formed cavity is called the archen- 

 teron, which will eventually become a part 

 of the digestive tract of the adult animal. 

 The two layers of cells that make up the 

 gastrula are called germ layers, the one on 

 the outside is the ectoderm and the inner 

 one, the endoderm. Presently, tiny pouches 

 push out from the archenteron and pinch 

 off, forming a double layer of cells, the 

 mesoderm, with a cavity, the coelom, be- 

 tween (Fig. 23-2). The coelom finally 

 becomes the body cavity in the adult. In 

 higher vertebrates such as mammals the 



o 



coelom gives rise to the pericardial, pleu- 

 rial, and abdominal cavities of the adult. 

 The three germ layers are formed in 

 somewhat different ways in various animal 

 groups, owing primarily to the presence of 

 yolk in the egg. In the case of Amphioxus, 

 where there is very little yolk, development 

 proceeds in the manner described. How- 

 ever, when the egg contains a great deal of 

 yolk, as in the case of the bird's egg, the 

 formation of the three germ layers is modi- 

 fied, although the end result is essentially 

 the same. In the chick, which is best known 

 and most frequently studied (Fig. 23-4), 

 the embryo forms on top of the yolk mass. 

 The two-layered embryo or gastrula is pro- 

 duced by a splitting of the single-layered 

 embryo, and the mesoderm subsequently 

 appears between the ectoderm and endo- 

 derm, spreading laterally from a central 

 axis called the primitive streak. Segmental 

 blocks of mesoderm, the somites, appear 

 first in the anterior trunk region of the em- 

 bryo and continue posteriorly as growth 

 occurs. The embryo becomes organized an- 

 terior to the primitive streak. By this time 

 (33 hours) the cord and three divisions of 

 the brain can be seen, and the heart is 

 laid down. From these beginnings the em- 

 bryo body is formed. 



With the formation of these three germ 

 layers, the basic raw materials are present 



