REPRODUCTION AND DEVELOPMENT IN CHORDATES 



Since these cells are pigmented, the direction and extent of their movements 

 can be noted; the original location of the gray crescent can still be ob- 

 served. The establishment of the germ ring and the beginning of its shift in 

 position mark the end of that part of development in which cell division is the 

 characteristic event. Cell division, however, continues, and the number of 

 cells increases greatly during the next period of development. 



In eggs that do not have a natural marker such as the gray crescent of 

 some amphibian eggs, it has been possible to mark different parts of the 

 blastula by means of non-toxic or vital dyes. It has been found that cer- 

 tain limited areas of the blastula always move, in the normal course of 

 development, to a specific part of the gastrula. These regions whose develop- 

 mental fate, so to speak, is already determined at the blastula stage are 

 called presumptive areas. 



Cell Localization. The spread of cells formed at the original equatorial 

 region of the zygote toward the vegetal pole is the beginning of a phase of 

 development characterized by mass movements of cells which lead to the 

 establishment of the so-called body plan of the chordate. During this period 

 of cell localization masses of cells are brought into new relations with one 

 another. The embryo is presently found to consist of three distinguishable 

 layers of cells which have long been called the germ layers, namely, ectoderm, 

 endoderm, and mesoderm. The first part of cell localization is frequently 

 referred to as gastrulation. 



Gastrulation in the amphioxus is a much simpler process than in amphibia, 

 where yolk-laden cells are a complicating factor in the movements that occur. 

 In the amphioxus the late blastula becomes somewhat flattened at the vegetal 

 end, the cells of which begin to move as a group into the blastula cavity 

 (Fig. 5.\4E-H). This is known as the process of invagination. The open 

 end of the early gastrula, as the developing individual is now called, is known 

 as the blastopore, and the cells of the germ ring form its lips. The move- 

 ments of the cells continue until the blastula cavity is obliterated completely 

 and there is a new internal cavity, the gastrula cavity, gastrocoel, or archenteron, 

 which opens externally by way of the blastopore. As a result of the shifting 

 of cells the individual now consists of two layers of cells, an outer layer of 

 ectoderm and an inner layer containing the primordia of the other two germ 

 layers, which will be sorted out by subsequent movements. The outer and 

 inner layers are continuous with one another in the region of the germ ring, 

 that is, at the lips of the blastopore. Continued cell divisions add cells to 

 both these layers, and the gastrula becomes elongated as the germ ring de- 

 creases in circumference. This decrease in size of the germ ring is also known 

 as the closure of the blastopore, which becomes smaller and smaller. This 

 stage in the development of chordates is suggestive of the so-called diploblas- 

 tic, or two-layered, body plan of some coelenterates (pp. 226 and 284). 



Gastrulation in amphibia is essentially the same as in the amphioxus. The 

 yolk-laden cells tend to move toward the blastula cavity, but invagination is 

 not the conspicuous process that it is in amphioxus (Fig. 5.15Z)). A more 



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