736 LIFE : OUTLINES OF GENERAL BIOLOGY 



lished — the outer ectoderm or epiblast, and the inner endoderm or 

 hypoblast. It was a great step, familiar enough now, when Huxley 

 definitely pointed out that the epiblast of an embryo corresponds 

 with the outermost layer or ectoderm of a two-layered (diplo- 

 blastic) animal, such as Hydra; and the hypoblast to the endoderm. 

 It is now convenient and usual to abandon the synonyms epiblast 

 and hypoblast, and keep to ectoderm and endoderm. 



Mesoderm. — In Sponges and Coelentera there are only two well- 

 defined layers, the ectoderm and the endoderm, which are separated 

 from one another by a gelatinous material (mesogloea) into which 

 cells migrate. In the Ctenophore class of Coelentera, there is for the 

 first time a definite third layer — the mesoderm or mesoblast. Thus 

 from this level onwards animals are called triploblastic, in contrast 

 to the diploblastic sponges and Coelentera. 



The origin of the mesoderm is diverse and often difficult, though 

 in the development of some types, e.g. Arrow-worm (Sagitta) and 



Fig. 1 1 8. 



The Typical Process of Gastrulation, as in Sea-Urchin. A hollow ball of cells, 

 a blastula (i), becomes through inequalities of growth (2) indimpled or 

 invaginated. The result is a gastrula, with two layers of cells, ectoderm 

 to the outside and endoderm to the inside. The cavity of the gastrula, 

 the archenteron, becomes the digestive cavity. The opening of the 

 gastrula is called the blastopore. 



Lancelet (Amphioxus), its mode of appearance is beautifully clear. 

 It arises in the main in the form of pouches from the archenteron, 

 which lose their connection with their origin, and form the body- 

 cavity or coelom between the gut and the body-wall. The inner 

 (splanchnic) wall of a typical coelom-pouch enswathes the food- 

 canal or gut; the outer (somatic) wall chngs to the ectoderm and 

 forms the under-skin or dermis; the dorsal portions of the pouches 

 form the bulk of the body-musculature. 



In some other cases the outgrowths or pouchings are solid to start 

 with. In the earthworm's development the mesoderm is due to a 

 few primary "mesoblasts", cells which appear at an early stage 

 between ectoderm and endoderm and multiply there to form 

 muscular tissue and the like. In many cases, as in Echinoderms, 

 there are, in addition to pouches, numerous "mesenchyme" cells, 

 separated off individually from the walls of the blastula or gastrula. 

 In almost all animals that have a cavity between the gut and the 

 body-wall, the mesoderm lines that cavity. 



