66 REPRODUCTION AND LIFE HISTORY. 



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 emboli (Fig. 30). 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 epiboli. 



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. 

 148.) 



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 less distinct than in higher forms, and 

 is usually represented by a gelatinous material (mesoglxa) 

 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 

 mesoblasts or cells which appear at an early stage between 

 the ectoderm and endoderm {e.g. in the earthworm's 

 development) ; or from numerous " mesenchyme " immi- 

 grant cells, which are separated from the walls of the blastula 

 or gastrula (e.g. in the development of Echinoderms) ; or 

 as caelom pouches — outgrowths from the endodermic lining 

 of the gastrula cavity (e.g. in Sagitta, Balanoglossus, 

 Amphioxus) ; or by combinations of these and other modes 

 of origin. The mesoderm lies or comes to lie between 

 ectoderm and endoderm, and it lines the bodycavity, one 

 layer of mesoderm (parietal or somatic) clinging to the 

 ectodermic external wall, the other (visceral or splanchnic) 

 cleaving to the endodermic gut and its outgrowths. 



Origin of organs. — From the outer ectoderm and inner 

 endoderm, those organs arise which are consonant with the 

 position of these two layers, thus nervous system from the 



