MESODERM. 67 



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. 



Gaitrula. — 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 aji 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 einbole. 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 

 described 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 

 containing cells, and a gastrula is formed by overgrowth or 

 epibole. 



In the course of our studies, we shall have opportunity to 

 discuss various forms of gastrulation, and some other pro- 

 cesses by which two layers are established, such as that 

 called delamination. 



Alesoderm. — 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 Ccelentera it is less distinct than in higher forms, 

 and is usually represented by a gelatinous material {meso- 

 glaia) which appears between ectoderm and endoderm, and 

 into which cells wander from these two layers. In the 

 other Metazoa, the middle layer may arise from a few 

 primary inesoblasts or cells which appear at an early stage 

 between the ectoderm and endoderm {e.g., in the earth- 

 worm's development) ; or from numerous " mesenchyme " 

 immigrant cells, which are separated from the walls of the 

 blastula or gastrula {e.g., in the development of Echino- 

 derms) ; or as caloine puiiches — outgrowths from the endo- 

 dermic lining of the gastrula cavity {e.g., in Sagitta, Balano- 

 glossus, Amphioxus) ; or by combinations of these and other 

 modes of origin. The mesoderm lies or comes to lie be- 



