GASTRULA 8 1 



containing lithium sea-urchin eggs fail to gastrulate 

 properly ; no invagination takes place, and abnormal hour- 

 glass-shaped larvae are produced. 



Blastosphere and morula. — The result of the division 

 is usually a ball of cells. But when the yolk is very 

 abundant a disc of cells — a discoidal blastoderm — is 

 formed at one pole of the mass of nutritive material, which 

 it gradually surrounds. 



As the cells divide and redivide, they often leave a large 

 central cavity — the segmentation cavity — and a hollow ball 

 of cells — a blastosphere or blastula — results. 



But if the so-called " segmentation cavity " be very small 

 or absent, a solid ball of cells or morula, like the fruit of 

 bramble or mulberry, results* 



Gastrula. — The next great step in development is the 

 establishment of the two primary germinal layers, the outer 

 ectoderm and the inner endoderm, or the epiblast and the 

 hypoblast. 



One hemisphere of the hollow ball of cells may be appar- 

 ently dimpled into the other, as we might dimple an india- 

 rubber ball which had a hole in it. Thus out of a hollow 

 ball of cells, a two-layered sac is formed — a gastrula formed 

 by invagination or embole (Fig. 40). The mouth of the 

 gastrula is called the blastopore, its cavity the archenteron. 



But where the ball of cells is practically a solid morula, 

 the apparent in-dimpling cannot occur in the fashion de- 

 scribed above. Yet in these cases the two-layered gastrula 

 is still formed. The smaller, less yolk-laden cells, towards 

 the animal pole, gradually grow round the larger yolk-con- 

 taining cells, and a gastrula is formed by overgrowth or 

 epibole. 



In various ways the ectoderm and the endoderm are 

 established, either by some form of gastrulation, or by some 

 other process, such as that called de lamination. 



Mesoderm. — We are not yet able to make general state- 

 ments of much value in regard to the origin of the middle 

 germinal layer — the mesoderm or mesoblast. In Sponges 

 and Coelentera it is not a distinct layer except in Cteno- 

 phora, being usually represented by a gelatinous material 

 {mesogloea), which appears between ectoderm and endoderm, 

 and into which cells wander from these two layers. In the 



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