6o REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



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 

 indiarubber 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. The mouth of the 

 gastrula is called the blastopore, its cavity is 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 as to some other 

 processes by which two layers are established, such as that 

 called delamination. These are so rare that I need not here 

 take any account of them. 



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 Coelenterates there is really none, but a gelatinous stuff 

 (mesoglcea) appears between ectoderm and endoderm, and into 

 this there wander cells from these two layers. In the other 

 Metazoa, the middle layer may arise from a it.v^ primary meso- 

 blasts or cells which appear at an early stage between the ecto- 

 derm and endoderm (e.g., in the earthworm's development) : 

 or from numerous " mesenchyme " immigrant cells, which are 

 separated from the walls of the blastosphere or of gastrula 

 (e.g., in the development of Echinoderms) ; or as ccelome 

 pouches — pocket-like outgrowths from the endodermic lining 

 of the gastrula cavity {e.g.., in Sagitta, Balanoglossus, Am- 

 phioxus); or by combinations of these and other modes of 

 origin. The mesoderm lies or comes to lie between ectoderm 



