BLASTULA AND GASTRULA. 67 



(4) Eggs with a considerable quantity of yolk accumulated in a 

 central core and surrounded by the formative protoplasm, 

 divide partially, and superficially or peripherally, 

 [or, Ova which are centrolecithal undergo meroblastic and super- 

 ficial segmentation]. 



This is illustrated by most Arthropods (Fig. 31 (4^), and 

 by them alone. 



Blastosphere and monila. The result of the division is 

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

 (3), 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. 32). 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 

 epibolL 



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 delamination (see p. 163). 



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 



